Subversion Repositories eco32

Project History
---------------

14-Nov-2002
Project launched.
Main makefile written.
Directory structure created.

15-Nov-2002
Kernel of UNIX 7th Edition copied.
Download and installation of lcc.
This included the following steps:
1) In 'src', copy 'mips.md' to 'eco32.md' without changes,
   except that the interface record is named 'eco32IR'.
   The code output remains for now as it is in 'mips.md'.
2) Add instructions to makefile for compiling 'eco32.md'.
3) In 'src', add the interface record definition 'eco32IR'
   to the file 'bind.c'.
4) Add a directory 'include/eco32/linux' and copy all header
   files from the corresponding directory for the mips processor.
5) In 'etc', copy 'irix.c' to 'eco32-linux.c' and change the
   contents of this file to properly call the preprocessor,
   the compiler, etc.
6) Add the option -DLCCDIR=\"$(BUILDDIR)/\" to the instruction
   in the makefile which compiles $(HOSTFILE).
7) In 'src', substitute 'lex.c' with a newer version, which
   computes the types of hexadecimal constants correctly.
Disk creator done.
Program to write the bootstrap sector done.

16-Nov-2002
Preliminary instruction formats defined.
Preliminary instruction set constructed.
Some ideas for an address translation unit noted.

17-Nov-2002
Final instruction formats defined.
Preliminary version of simulator finished.

19-Nov-2002
Instruction set cleanup done.
Work on assembler started.

23-Nov-2002
Four of the five ROMs translated to ECO32.

24-Nov-2002
Preliminary version of assembler/linker/loader finished.

25-Nov-2002
Bootstrap monitor ROM translated to ECO32.
Simulator is running.
Bootstrap sector on disk created.
Now it is possible to boot from the disk.

26-Nov-2002
Work on an ECO32 back-end for the C compiler started.

27-Nov-2002
Preliminary version of back-end done.

28-Nov-2002
'Load address' instruction invented as macro for a set
of short sequences of instructions which are difficult
to generate by the back-end.

29-Nov-2002
Startup code (c0.s, c1.s) revised.
Writing the bootstrap sector revised.
System ROMs revised.
Tests revised.

30-Nov-2002
Started to program the MMU.
Surprise: we don't need an MMU enable bit! The unit can be
switched on all the time, but we have to be careful not to
use the TLB before it is properly initialized.

01-Dec-2002
Interrupt system re-designed.
Added the last missing case in the 'lda' implementation.
Timer interrupt is working properly.
We now have a new instruction: 'nor'. With this it is easy
to implement the bitwise complement of an integer. Done.
The compiler option -Wo-kernel compiles a program so that
it can be run in kernel mode.
This is a big step forward: hello.c is running (in kernel
mode of course).

02-Dec-2002
The characteristic feature of kernel mode programs as well as
of the kernel itself is that they want to handle all sorts of
special conditions for themselves (e.g., interrupts, exceptions,
startup code). So if the flag -Wo-kernel is given, the startup
library code (c0.s and c1.s) is no longer linked in automatically.

03-Dec-2002
The assembler now contains a complete expression parser which can
handle additive, multiplicative and shift expressions as well as
unary and parenthesized expressions.
Error in back-end corrected: mul & div instructions did not
correctly specify the second source register.

04-Dec-2002
The low-level code generators in the assembler had completely to
be redone: they could not handle big constants (immediate values
and addresses) in instructions that allow only 16 bit constants.

05-Dec-2002
I now have a pair of functions which allow to get/set an
interrupt service routine in the interrupt service routine
table. The low-level register save/restore is handled in
assembler, but the interrupt service routine itself can be
written in C. An application of this principle shows how to
determine the total amount of physical memory by probing
the memory until a bus timeout exception occurs. This can
fully be formulated in C.
The back-end generates a 'jal' instruction even when the
argument is a register. Although it would perhaps be more
appropriate to generate a 'jalr' instruction, I changed the
assembler to accept an address as well as a register with
this instruction.

06-Dec-2002
TLB flush is the first instruction dealing with the TLB.
Implemented.

07-Dec-2002
Until now we only had one special processor register, the
processor status word. With the MMU present, this will change.
Renamed 'gpsw' (get processor status word) to 'mvfs' (move
from special register). Renamed 'ppsw' (put processor status
word) to 'mvts' (move to special register). These instructions
have two arguments: a single register and a number specifying
the special register.
It is perhaps better to use an established set of instructions
dealing with the TLB (i.e., the way MIPS is doing this) instead
of an even more minimalistic one, which is not guaranteed to
cover all cases. So I will abandon 'TLB flush' in favor of the
four instructions found in the MIPS instruction set ('tbs',
'tbwr', 'tbri', and 'tbwi').
As is documented in the MIPS reference book, the valid bit V is
not involved in the TLB matching process. Implementation changed.
Perhaps we don't need the V bit at all? I'll leave it out for now.

08-Dec-2002
Today I came up with the idea to test the MMU implementation by
running a somewhat more realistic test: a task which is compiled
for virtual address 0 and has a minimal I/O library which traps
to the kernel to output a character.

09-Dec-2002
It is not necessary to prepare real page tables for the task, as I
thought initially. Since we have only one task which uses exactly
two pages (code and stack), it is possible to preset the TLB with
two entries for the pages and then leave it alone.
The test is running.

10-Dec-2002
Now for the next step: two tasks running concurrently, preempted
by a timer interrupt. This is really a lot more complicated than
a single task because now the entries in the TLB must be flushed
and re-programmed when a task switch occurs.
Another complication is the need for two kernel-mode stacks, one
for each task, in addition to the task-specific user-mode stacks.
This is necessary because of two facts:
  - the kernel cannot rely on the integrity of the user-mode stacks
  - the state of the suspended task must be preserved somewhere,
    even if the other task is running and trapping to the kernel
    for a system call.

11-Dec-2002
This was indeed much harder than expected, but now it works.

12-Dec-2002
It is evident that only a very short sequence of instructions is
able to handle a TLB miss exception efficiently. MIPS is doing
this by giving the TLB miss exception a separate interrupt vector.
We don't want to go so far, but of course we cannot tolerate to
execute all the lengthy actions at the beginning of the standard
trap handler. So we will sort out the TLB miss exception at the
very beginning of the standard exception handling.

13-Dec-2002
It is easy to catch the TLB miss - but how should it be handled?
Some sort of page table has to supply the necessary information
so that a new entry can be filled into the TLB. How should the
page tables be organized? In the past I saw cascaded page tables
as the only solution. But it might be better to have a single
page table which itself is paged and lies in virtual memory.
The big advantages of this scheme are fast access (no pointers
to chase) and (almost) no waste of memory because gaps in the
table do not need any real memory. But then of course we have
to handle TLB misses while accessing the page table...
Very confusing!

14-Dec-2002
What information is needed in which format if a TLB miss (or any
other TLB exception) occurs? I really have no stringent answer to
this question, so I will follow again a minimalistic design path:
Implement a register in the MMU which is set to the offending
virtual address in case the translation didn't work. Done.
As I realize now, this hardware equipment can support a variety of
different organizations of the page tables handled by software.
Even inverted page tables seem feasible!
It would be very convenient if the kernel could use another
register without having to save its contents first. I think
we could let the kernel have also register 28 at its disposal.

17-Dec-2002
Yesterday I realized that the 'lda' macro instruction is no longer
needed because 'add' does the very same job. Deleted.

18-Dec-2002
It is very desirable to have an interrupt mask in the PSW.
If, e.g., the timer interrupts and the timer ISR decides to
disable interrupts from all devices but the timer itself,
then this is not easily achievable without a central mask.

19-Dec-2002
One's complement, represented by '~', is another operator
implemented in the assembler.

20-Dec-2002
As well as bitwise and, bitwise xor, and bitwise or,
represented by '&', '^', and '|', respectively. These
operators are left associative and have the same priority
as they have in C, i.e., & > ^ > |, in order of decreasing
priority.

21-Dec-2002
Because of the new structure of the PSW all tests must be
updated and carried out once more. A lot of work...

22-Dec-2002
I spotted a minor bug in the simulator: a breakpoint which
was set to address C0000004 in order to catch any interrupt
or exception will not work for exceptions. This is due to
the handling of exceptions by setjmp/longjmp. Corrected.

23-Dec-2002
ECO32 Version 0.0 released.

26-Dec-2002
Today I started to convert the original sources of
UNIX 7th Edition to ANSI C.

29-Dec-2002
I found a real programming error in the text.c module:
  if (ip->i_flag&ITEXT==0) return;
My compiler warned: 'expression with no effect elided'.
'==' has higher precedence than '&', but ITEXT does not
equal zero, so the whole test expression becomes zero
and the return is never executed. The line should read:
  if ((ip->i_flag&ITEXT)==0) return;

31-Dec-2002
The masters made a typoe: in the file kl.c, the function
named 'ttioccomm' is spelled 'ttioccom'. I think this
did go undetected because of the very restricted length
of symbols with external linkage.

01-Jan-2003
Today I began investigating the startup procedure. I will
place the per-process kernel stack in the top half of the
u area, which itself is one page in size and is located
directly underneath the kernel at 0xC0000000 - 0x1000.
This part of the virtual address space is mapped and so
the u area of the running process is always accessible.

02-Jan-2003
How do we do the memory management? This is THE central
question of our porting effort. One possibility which
offers the chance of only minor changes to the existing
sources would be to keep the management by the coremap
array, but with the granularity changed to 4 k, the page
size. This route of course would not barely touch the
power of demand paging, which is possible by our hardware.
Nevertheless, I will follow this path for the time being.
I changed the coremap/swapmap entries into structs of two
integers instead of two shorts. Otherwise the range of
addresses might get too small (although this is unlikely:
64 k pages of 4 k bytes each equals 256 M bytes - quite a
lot of memory).

04-Jan-2003
This last change is nonsense - much of the other stored
information about processes would have to be changed also.
Reverted.
Because ECO32 does not have two different stack pointers
(as the PDP-11 has) we must switch to the kernel stack
explicitly. But we must not do the switch if the interrupt
hit while the machine was running in kernel mode already -
then the sp points deep into a properly setup kernel stack.
Switching stacks would then destroy the stack completely.

11-Jan-2003
I discovered a mysterious overwriting of some bytes of
the loaded UNIX kernel. This took place even before any
of its code was executed! A little bit of searching, and
the guilty one was found: the bootmon ROM code and its
stack. Now there is a problem: where should we put the
stack of the bootmon ROM?
We have two ways to get some program into the bare machine:
a) The 'realistic hardware' scenario: a ROM holds an initial
program which e.g. may boot the operating system from disk.
b) The 'convenient debugging scenario': the program, e.g. the
operating system, is somewhat magically loaded into RAM.
The error described above is caused by the unwanted interaction
of a) and b) when used together. So I think that the correct
solution of the problem is to forbid the specification of both
simulator options -r and -l at the same time.

13-Jan-2003
The simulator uses all CPU power which is available, even
when only waiting for a command line. This can easily be
changed by inserting a usleep(100) call into the input
waiting loop of the curses interface. Done.
Two new pseudo-ops (.syn and .nosyn) added to the assembler.
They allow or disallow synthesizing instructions with 'big'
constants, respectively. This can be used e.g. in the TLB miss
handler, where much finer control over the usage of register
$1 is needed.

15-Jan-2003
The constant BSLOP in param.h must be set to 0. Otherwise there
would exist blocks which don't start at an address which is a
multiple of 4. This would result in a Bus Address Error if e.g.
an integer is fetched from the block.

16-Jan-2003
The type mapping from C to machine types is as follows:
         PDP-11                   ECO32
char     byte (1 byte)            byte (1 byte)
short    word (2 bytes)           half (2 bytes)
int      word (2 bytes)           word (4 bytes)
long     double word (4 bytes)    word (4 bytes)
T *      word (2 bytes)           word (4 bytes)
As can be seen, only ints and pointers are differently mapped.
This is of not much concern with in-memory data structures but
can be desastrous with on-disk data structures (super block,
inodes). Therefore we have to do the following:
a) define ino_t as unsigned short
b) rearrange the components in struct filsys (and in principle in
struct dinode and struct direct too) so that no padding occurs.
I thought that I could simply write a disk with the PDP-11 simulator
and then use it with the ECO32 simulator; this is not possible
due to different endianness.

17-Jan-2003
Here are the changes I actually made:
1) in param.h:
   a) type 'ino_t' is 'unsigned short' instead of 'unsigned int'
   b) type 'dev_t' is 'short' instead of 'int'
2) in filsys.h:
   a) delete all components of struct filsys below the comment
      'remainder not maintained by this version of the system'
   b) 's_isize' is of type 'daddr_t', not of type 'unsigned short'
   c) 's_nfree' is of type 'int', not of type 'short'
   d) 's_ninode' is of type 'int', not of type 'short'
   e) exchange components 's_isize' and 's_fsize'
3) in inode.h:
   a) 'i_flag' is of type 'short', not of type 'char'
   b) 'i_count' is of type 'short', not of type 'char'
The size of the superblock structure is thus increased by 6 bytes
to 424 bytes (well below the 512 bytes block boundary), and all
padding or misalignment is avoided.

18-Jan-2003
I replaced the "Hello, world!" message in the default master boot
record by a somewhat more appropriate message.

19-Jan-2003
In order to simplify things a bit, I removed the '-n' option from
the assembler. Now assembler output files always do have headers.
There are of course situations in which the header must be stripped
off, e.g. with the boot record, or with the ROM in the simulator.
We have therefore now a 'loader', a little program which converts
an assembler output file into a memory image without any header.
This has the additional benefit that the simulator does no longer
need to know the executable format, which is especially good if we
change the format or add new formats.

20-Jan-2003
Started to port the 'mkfs' utility.

21-Jan-2003
Started to program a 'show file system' utility.

22-Jan-2003
Today I decided to spare a small number of TLB entries (4) from
random replacement. This helps to keep such vital information as
the current kernel stack permanently mapped.

25-Jan-2003
I changed the disk so that it is now similar to the 'rp' device
in the PDP-11. This has the advantage that the usr file system
is located on a different device (specifically: a different minor
device, i.e., a different partition) so that mounting can be tested.
The partitions and their intended use is as follows:
   device      size in blocks     use as
  -------------------------------------------
    rp0           14000           whole disk
    rp1            2000           root
    rp2            4000           swap
    rp3            8000           usr

28-Jan-2003
This was quite a lot of work, but finally 'mkfs' is running.
The crucial tool to achieve this was 'shfs' - show file system,
which allows displaying arbitrary blocks of a disk in several
formats (raw data, super block, inode block, directory block,
free list block, or indirect block). So everything on the disk
can be examined in detail.
By the way, the main hurdles to overcome had been different
endianness of ECO32 and x86, different padding of structures,
and alignment restrictions with ECO32, which are not present
on the x86 architecture.

29-Jan-2003
Here is a short report on byte-ordering on x86, ECO32, and PDP-11.
The C data type 'long' is stored in 4 bytes on all thre machines,
as already stated above (16-Jan-2003). The order of the bytes is
different on all three machines. We number the bytes from MSB
(most signigicant byte) to LSB (least significant byte): then
the number (b1 b2 b3 b4) has the value b1*2^24+b2*2^16+b3*2^8+b4.
If 'A' is the lowest (byte) address where the number is to be stored,
then the memory layout of the three architectures look like this:
x86       A+0:b4  A+1:b3  A+2:b2  A+3:b1    (i.e., little endian)
ECO32     A+0:b1  A+1:b2  A+2:b3  A+3:b4    (i.e., big endian)
PDP-11    A+0:b2  A+0:b1  A+0:b4  A+0:b3    (i.e., mixed)
An additional complication results from the fact that disk addresses
in inodes on a disk are stored in a three byte format, which is
different on ECO32 and PDP-11 (there is no such format on the x86,
because we don't want to run a file system on a bare PC, only within
the ECO32 simulator). In this format only the three least significant
bytes (b2 b3 b4) are stored; if b1 were non-zero an error would be
triggered. So then, here are the layouts of this special format:
ECO32     A+0:b2  A+1:b3  A+2:b4
PDP-11    A+0:b2  A+1:b4  A+2:b3
As can be seen, this format is constructed on both architectures by
omitting b1 (the MSB) from the four byte format.
I changed the routines that convert the three byte format from and to
the four byte format (iexpand() and iupdat(), both in module iget.c)
in order to cope with the changed byte-ordering.
Reduced the RAM size to 1M; this fits better to the disk swap size.
Added a counter to the simulator which counts the total number of
instructions executed since the last reset. This helps a lot if one
wants to re-visit a specific location in the program, if a lot of
single-stepping has been done to find the location for the first time.

31-Jan-2003
There has been an inconsistency with the object file format since
its invention: the header data is stored in little endian format,
which is convenient for the tools working in the cross development
setup. The tools running on the target architecture require the
data to be in ECO32 format of course. This is especially true for
the kernel exec file loader. Changed.

03-Feb-2003
ECO32 Version 0.1 released.

04-Feb-2003
Added to 'shfs' a command which translates an inode number into the
block number in which this inode is located, together with an inode
number relative to that block.
The simulator's 't' command now displays also the special registers
of the MMU (Index, EntryHi, EntryLo, BadAddress).
Memory commands of the simulator completed.

05-Feb-2003
ATTENTION: I changed the output of the disassembler to get it more
consistent with the rest of the simulator, and especially with its
inline assembler. There are no more '0x' prefixes in front of numbers,
and we don't have any decimal output. All numbers are given as hex
numbers without a prefix (the only exception to this rule are the
numbers of registers proper, of course, which are given in decimal).
The instructions which do sign extension show their arguments as
signed hex numbers. Labels are also no longer prefixed.
Inline assembler completed.

17-Feb-2003
ECO32 Version 0.2 released.

20-Aug-2003
ECO32 Version 0.3 released.

21-Aug-2003
1) I changed the 'ldhi' opcode from 0x1E to 0x1F.
   Reason: The instruction decoding gets more regular.
2) The ALU has now an 'xnor' instruction instead of a 'nor':
   (a xnor b) = ~(a ^ b) = (~a ^ b) = (a ^ ~b) = (a == b),
   bitwise parallel for all 32 bits.
   Reason: The ALU is easier to implement, and the instruction
   set becomes more regular.
3) The PC relative jumps (j, jal, all conditional branches) now
   have PC+4 as their reference location (and no longer PC).
   Reason: PC+4 is computed early in the instruction cycle; all
   other RISC machines do this alike.
4) The PC relative jumps (j, jal, all conditional branches) now
   encode a word offset, and no longer a byte offset.
   Reason: The target ranges for the jumps are quadrupled without
   any additional effort.

07-Oct-2003
Explored the possibilities to implement a graphics controller for
ECO32. Easy from the simulator's point of view, not quite so easy
from the implementer's point of view (due to X complications,
threads needed) but certainly feasible.

13-Oct-2003
Graphics controller implemented.

20-Oct-2003
Curses feeds certain keypad codes directly into the simulator's
terminal, e.g. KEY_BACKSPACE. There the MSBs get stripped off and
a wrong keycode is supplied to the simulated program. Now these
special codes are mapped to regular control characters, e.g. BS.

25-Oct-2003
The assembler now understands the option '-h' which suppresses the
output of the executable's header and appends the BSS's size bytes
of zeroes at the end of the executable. This in turn renders the
'load' tool redundant, which can be deleted. Done.

26-Oct-2003
A group of students is working on a ROM-based monitor program. They
need an instruction to implement breakpoints. Because the instruction
map is already crowded they have to use 'trap'. But they want of
course a breakpoint trap be easily distinguishable from any other
kind of trap. So the assembler now allows a constant operand for
'trap'. This operand is assembled into the otherwise unused bits
of the instruction. These bits are in turn ignored by the CPU (as
was the case all the time) but can be inspected by software which
must read the instruction from memory in order to do this. Now a
breakpoint trap can get a unique bit pattern distinct from the bit
patterns of all other traps. Remark: not only the regular assembler
must understand this but also the simulator's inline assembler as
well as its disassembler. Done.

09-Nov-2003
The curses library correctly sets the terminal mode to "raw" so that
neither SIGINT nor SIGQUIT signals can be generated by the keyboard.
With a graphics controller attached it is however possible that the
user selects "kill" from the window menu of the monitor window and
thus shuts down the connection to the X server. This in turn leads
to an uncontrolled instant termination of the whole simulator which
cannot be tolerated (e.g. the terminal would stay in raw mode: very
annoying for an unsuspecting user). Solution: install an I/O error
handler which gets called from the X system in case the connection
to the server is lost. Done.

16-Nov-2003
I added a check to the initialization of the memory simulation ensuring
that neither ROM image files nor program files loaded into main memory
can overflow the available space (and thus crash the simulator).

18-Nov-2003
The terminal simulation now handles the following control characters:
(input)    <return>         0x0D
           <backspace>      0x08
           <arrow down>     0x0E
           <arrow up>       0x10
           <arrow left>     0x02
           <arrow right>    0x06
(output)   0x07             beep
           0x0D             return cursor to begin of current line
           0x0A             scroll if cursor is in last line,
                            set cursor to next line in same column
           0x02             move cursor left (stop at leftmost column)
           0x06             move cursor right (stop at rightmost column)
           0x10             move cursor up (stop at topmost line)
           0x0E             move cursor down (stop at bottom line)
           0x08             delete character to the left of cursor,
                            move cursor left
           0x09             output spaces (minimum is one space),
                            until column mod 8 = 0

19-Nov-2003
Today I took the first few steps towards a bootable system disk.
Changes in bootstrap monitor:
  - 'g' command deleted
  - monitor stack sits just below 1M
  - master boot record loaded at physical address 0x00000000
  - master boot record started at virtual address 0xC0000000
Changes in constructing the default master boot record:
  - relocate code to 0xC0000000

20-Nov-2003
Now here is the big picture of the boot process:
a) ROM bootstrap
   The ROM loads the first sector of the disk and starts whatever it
   has loaded (provided that the signature is present). Until now this
   was always the default master boot record. In the future this will
   be the first stage bootstrap. This part is already finished.
b) First stage bootstrap
   The first stage looks for a file named "boot" in the root directory
   of the file system on the boot disk. This part must fit into a single
   sector and thus is restricted in its capabilities:
     - boot must lie in the root directoy
     - it must be one of the first 320 directory entries in the root
       directory (only the 10 direct blocks of the directory file are
       searched for it)
     - it must not be bigger than 5120 Bytes (only the 10 direct blocks
       of the executable file are loaded)
c) Second stage bootstrap
   This stage loads the kernel, perhaps asking the user for its name.

22-Nov-2003
First stage bootstrap completed.

27-Nov-2003
Second stage bootstrap completed.
It was indeed possible to ask the user for the path to the kernel he
wants to get started. The only kernel available for now is one of the
tests from tst/os.

29-Nov-2003
Makefiles tweaked.

06-Dec-2003
Today I started to develop the tools shfs and mkfs for file systems
with 4K blocksize. With this blocksize it should be possible to do
the bootstrap without a named second stage. Conceptually there are
again two stages (first load a single sector, then the rest), but
both stages should fit within the boot block.

20-Dec-2003
I discovered a small bug (?) in the simulator: if a write to $0 is
requested by an instruction, not only the write is suppressed (this
is expected) but also the source operand does not get computed.
Thus the instruction ldw $0,$0,1 which is incorrect if no TLB entry
for page 0 exists (and is so even if it exists because of alignment
violation) does not lead to an exception (which is quite unexpected).
Solution: Always compute the source operands, even if $0 is the target.

30-Dec-2003
In order to use swap space on the same disk where the root file system
is located we need some form of disk partitioning, preferrably NOT
deeply coded into the disk driver (as the original UNIX has done it).
In other words, we need a real master boot record, a partition table
and a tool ("fdisk" or "mkpart") to set up such a table.

01-Jan-2004
"mkpart" finished. It constructs a partition table from a textual
configuration file and writes it to sector 1. It also copies an MBR
(which has not been done yet) to sector 0, and a bootstrap manager
(which also has yet to be done) to sectors 2-7.
MBR coded.

02-Jan-2004
Bootstrap manager done.

03-Jan-2004
Tool "mkpart" is working.
New tool "shpart" displays a partition table, read from a disk image.

04-Jan-2004
I made the file systen viewer aware of partitions.

05-Jan-2004
Now "mkfs" also knows something about partitions.

08-Jan-2004
If the operating system is booted from a partition of a disk (in
contrast to being booted from the start of the whole disk) its disk
driver must know where logical sector 0 of the partition is located
on the disk. In other words, the master bootstrap must transfer a
little bit of information to the boot loader of the OS. This could
be done as follows: $26 holds the start sector of the partition from
where the OS is booted and $27 holds its size. In order to use the
very same bootstrap also in the case that the disk has no partitions
(and thus the file system occupies the whole disk), this information
must also be supplied by the ROM bootstrap sequence. Done.
Surprise: this scheme would allow the sub-partitioning of a partition
without any modification!

10-Jan-2004
Now the whole bootstrap chain is working. It goes like this:
a) The ROM loads the first sector of the disk and starts whatever it
   has loaded (provided that the 0x55AA signature is present). This
   may be either the master boot record if the disk is partitioned,
   or an ordinary boot record if it is not (see remark below).
b) In case the master boot record is executing, it loads sectors 2..7
   into memory (sector 1, the second sector of the disk, is skipped
   because it holds the partition table). These sectors contain a small
   program, the boot manager, in executable format. The boot manager is
   started by the master boot record.
c) The boot manager loads the partition table from sector 1 of the disk,
   displays the partitions of the disk, and asks the user to enter a
   partition number to boot. An attempt to boot a partition outside the
   range 0..15 is rejected, as well as requesting to boot a partition
   which does not contain a file system of any kind, or does not have
   the "bootable" flag set. If all is well, the boot record of the
   selected partition is loaded. If it has a valid 0x55AA signature,
   it is started.
   Remark: This is the point in the bootstrap chain which is reached
   immediately from the ROM code if the disk is not partitioned.
d) The boot record code loads partition relative sectors 1..7, i.e.
   the rest of the boot block. These sectors contain a small program,
   the bootstrap loader, in executable format. The bootstrap loader is
   started by the boot record code.
e) The bootstrap loader asks the user for a path to a file which must
   exist, must be a regular file, and must be executable by its owner.
   This will of course be normally an operating system kernel. The
   default value is /boot/unix. The file is loaded and started.
Throughout the whole process registers $26 and $27 hold the start sector
and size of the currently selected range of the disk, respectively.

12-Jan-2004
The register conventions shown above must be changed somewhat because
with more than one disk present it is also necessary to tell the disk
driver which disk to use:
$26 base address of disk controller
$27 start sector of partition (or disk)
$28 size of partition (or disk) in sectors

28-Jan-2004
After rewriting parts of the make file system utility (so that double
indirect blocks can be used, which is necessary for a file system with
512 byte blocks to hold files of more than 69K) it is now possible to
boot our port of UNIX 7th Edition from the disk. Alas, it is not yet
running very stable.

10-Mar-2004
Finally I added the generation of copying loops to the compiler's
back-end. This has long been missing.

17-Apr-2004
Translating virtual addresses using the TLB costs time because the
parallel associative hardware must be simulated by serial software.
This is very noticeable with programs running in user mode; they run
at roughly one third of the speed kernel mode programs do. In order
to alleviate this I added a function which acts almost the same as
the TLB associative lookup does, but without any error checks. Its
sole pupose is to use up time and it is called whenever an unmapped
address is detected by the virtual-to-physical address translation.

21-Apr-2004
Up till now the assembler located the data segment directly above
the code segment. This is not a viable strategy once paging is in
effect. Due to code sharing and protection reasons, the data segment
should start on the next page boundary above the code. The resulting
gap may also be present in the executable file: there is no need to
store any bytes for it in the executable (at least as long as it is
an executable with a header - for headerless executables the gap
bytes must be stored within the file).
It would have been possible to define two different executable formats,
one with the data segment directly following the code segment, and
another one with the data segment aligned on a page boundary. The
two formats then would have to be distinguished by different magic
numbers in the header. I did not do this, however, because I saw
no reason to keep the "directly following" format. If one wants to
compress code and data as much as possible, one can in any case put
both into the code segment.

24-Apr-2004
The simulated graphics device did not detect memory accesses which
lie within the device address bounds but outside the screen memory.
These accesses were propagated to the X Window System and in turn an
X Window data structure was accessed outside of its allocated memory.
At least on our Sun computer this translated to an immediate death
of the simulator process. Now the graphics device detects such an
attempt to access simulated memory outside the screen, and produces
a 'bus timeout' exception.

06-May-2004
ECO32 Version 0.4 released.

08-May-2004
I intend to give the ECO32 simulator the ability to run with more than
one simulated terminal. This will work best if there is no difference
between the first terminal and the other ones, so I will open a separate
window for each terminal (and use the first window in which the simulator
was started only for controlling the simulator, and NOT for a simulated
terminal). Then using curses is no longer necessary and can be removed.
But I want to retain command line editing, so I will use the 'getline'
library.
Curses interface removed.
Getline library installed.
Interrupt priorities changed:
  15    not used
  14    timer
  13    not used
  12    not used
  11    not used
  10    not used
  09    not used
  08    disk
  07    terminal 3 receiver
  06    terminal 3 transmitter
  05    terminal 2 receiver
  04    terminal 2 transmitter
  03    terminal 1 receiver
  02    terminal 1 transmitter
  01    terminal 0 receiver
  00    terminal 0 transmitter

10-May-2004
SIGINT handler installed (for use within the control window to regain
control in case of a runaway program).
In order to use a single timer callback routine for all terminals, the
callback routine must get an argument, telling which terminal is doing
the callback. So ALL timer callbacks now have such an argument, which
can be used arbitrarily by the callback initiator to communicate an
integer to the callback routine.

17-May-2004
The new terminal implementation must be tested as well as the changed
interrupt priority scheme. And even more important: allocation of the
data segment on a page boundary (see 21-Apr-2004) must be verified!
Done for the 'tst' directory.

18-May-2004
Done for the 'eos' directory.

27-Feb-2005
ECO32 Version 0.5 released.

28-Feb-2005
Despite the long time since the release of the previous version this
was very premature: I did not read the comments above and forgot to
conduct many of the tests. And indeed, the bootstrap does not work
any longer.

01-Mar-2005
The bootstrap manager should not be compiled to any standard executable
format, because this part of the boot process is independent of any
operating system. It should be compiled to plain binary format, as is
the master boot record. Now we are facing a dilemma: We want to write
the boot manager (mostly) in C and therefore will have a separate data
segment, aligned on a page boundary. But we also want a headerless
binary output, so there will be no loading information. All gaps in
the loaded program and data will therefore be filled with zeroes.
Then either we must define another executable format in which the data
segment follows directly after the code segment (without padding to the
next page boundary, see discussion above) or we have to spend somewhat
more space on disk for the bootstrap managers's program/data image.
I think I will do the latter.

04-Mar-2005
The solution above is not very elegant: The same reasoning applies also
to the bootstrap within a partition. Then we are forced to reserve two
blocks instead of the traditional single one for the bootstrap. This is
not totally out of question, but not very appealing either.
There is another solution, if we accept the boot manager to be compiled
to standard executable format: write a very compact loader for this format
and squeeze it into the master boot record. Difficult, but it can be
done, as I proved today.
Changes from above reverted.
Disk partitioning, master boot record, and boot manager done.

05-Mar-2005
Boot record and bootstrap loader done.

08-Mar-2005
I think that today I completed the tests and changes which I begun
on 17-May-2004.
ECO32 Version 0.6 released.

27-Oct-2005
ECO32 Version 0.7 released.
This was done in order to have a stable release on which to build
a simplified version of the ECO32, which is named ECO32e (the 'e'
stands for 'embedded'). The ECO32e will get implemented in an FPGA.

26-Jun-2006
We do have an error in the implementation of the SAR instructions,
really! The C standard says that the result of shifts with shift
amounts greater than or equal to (!) the number of bits in the left
expression's type is undefined. Consequently, in the CPU implementation
cpu.c we have to replace
smsk = (MSB ? 0xFFFFFFFF << (32 - scnt) : 0);
in the SAR and SARI instructions with
smsk = (MSB ? ~(((Word) 0xFFFFFFFF) >> scnt) : 0);
because if scnt == 0 then 0xFFFFFFFF << (32 - scnt) is undefined.
Thanks to Rolf Viehmann for pointing this out.

27-Jun-2006
The rfx instruction must not be executed in user mode, because it
writes the previous mode and interrupt enable bits of the PSW over
the current ones. Therefore rfx becomes a privileged instruction
which triggers EXC_PRV_INSTRCT if executed in user mode.
Again thanks to Rolf Viehmann for recognizing the potential danger
if this instruction is executed in user mode.

09-Jul-2006
Pending interrupts are now displayed when executing the 'register'
command. It is not meaningful to display pending exceptions, however,
because these are immediately handled within the longjump framework
and thus would never show anything else but 'off'.

10-Jul-2006
We have a new command to display (and possibly change) the value of
the PSW. This comes in very handy if debugging in user mode and thus
not having access to privileged addresses.

12-Jul-2006
There is a new control bit in the PSW, the 'vector bit' V, which directs
interrupts to ROM_BASE + 4 if it is off and to RAM_BASE + 4 if it is on.
This bit is cleared when the CPU is reset. In this way the software can
decide whether the interrupt service routine is located in ROM or in RAM.

15-Jul-2006
I decided to give "user TLB misses" an extra interrupt service routine
entry point. These are TLB misses triggered by an access to a virtual
address with MSB = 0, regardless of execution mode (user or kernel).

22-Jul-2006
The physical address space (2^30 bytes) is partitioned as follows
(in order to be compatible with the ECO32e):
RAM    virt start 0xC0000000    phys start 0x00000000    size 0x20000000
ROM    virt start 0xE0000000    phys start 0x20000000    size 0x10000000
I/O    virt start 0xF0000000    phys start 0x30000000    size 0x10000000
The I/O address space is evenly divided into 256 devices which may
occupy up to 1MByte each.

30-Jul-2006
We now have two control bits in the outgoing part of every entry of
the TLB: a "valid" bit (which triggers a "TLB invalid exception" if
this entry is selected by a memory translation and the bit is 0) and
a "write" bit (which triggers a "TLB write exception" if this entry
is used to map a write access to a page and the bit is 0). The "TLB
double hit exception" does not exist any longer.
The reason for existence of the "write" bit is obvious (possibility
to protect pages from writing, implement a copy-on-write policy).
The reason for existence of the "valid" bit is speed: the TLB refill
routine should run as fast as possible and therefore should avoid
tests for special cases as much as possible. If some processing must
be done for a page to be usable, the check for this condition should
not be done in the TLB miss handler. If the valid bit is cleared in
the page table, the refill routine will enter the translation that it
gets from the page table into the TLB without checking, but the next
use of this entry will lead to another exception, where the processing
then can be done.

31-Jul-2006
The implementation of the ECO32e suggested a different coding of the
single register operand in the jr and jalr instructions. It is now
coded in the src1 field instead of the dst field of these instructions.

01-Aug-2006
a) I rearranged the exceptions as follows:
EXC_BUS_TIMEOUT 16
EXC_ILL_INSTRCT 17
EXC_PRV_INSTRCT 18
EXC_DIVIDE      19
EXC_TRAP        20
EXC_TLB_MISS    21
EXC_TLB_WRITE   22
EXC_TLB_INVALID 23
EXC_ILL_ADDRESS 24
EXC_PRV_ADDRESS 25
In this way they are grouped together more logically.
b) The detection of an EXC_ILL_ADDRESS exception (formerly called
EXC_BUS_ADDRESS) is now done in the MMU and thus operates on logical
addresses. It is then possible to store the offending address in the
BadAddr register for further processing by OS software.
c) The EntryHi register now gets loaded with the page base address
if one of the three TLB exceptions occurs, so that it need not be
computed from the contents of the BadAddr register.

02-Aug-2006
A "context" register, which is preloaded by OS software with the base
address of memory-resident page tables, and where the page number of
a page with missing translation is ored-in by hardware would speed up
the TLB miss handler. On the other hand, it practically mandates the
use of a certain page table format (or, if the OS designer wants a
different format, the context register would not be used at all).
Therefore I will not implement a context register.

03-Oct-2006
Major instruction format redesign: the decision to put the destination
register address in a fixed location within the instruction was wrong.
It implied that the source register addresses were not fixed and thus
must be multiplexed early in the instruction cycle dependent upon the
opcode. It should be done the other way around. Completed.

19-Nov-2006
Today I completed the back-port of the display and keyboard from ECO32e.
Both are available if the command line switch -c ("console") is given.
The default number of terminals was in turn reduced to 0.

14-Feb-2007
Interrupts are now level triggered. If at any time interrupt sharing
would become necessary (this is in fact quite possible with only 16
interrupts) it cannot be realized if interrupts were edge triggered.

04-Jun-2008
Project finally split:
ECO32 (version 0.17) -- FPGA hardware, simulator, assembler
EOS32 (version 0.11) -- C compiler, ROM monitor, operating system

20-Feb-2009
ECO32 now has its own linkable object file format, which is a variant
of the venerable a.out format. Consequently, 'asld' has been split into
the assembler 'as' and the linking loader 'ld'. Additionally, there is
a program 'dof' which dumps the contents of an object file.

22-Feb-2009
What is the exact meaning of 'jalr $31'? This can of course be defined
arbitrarily, but the natural semantics would likely be "jump to the
routine addressed by $31 and remember the return address (in $31)".
To my surprise, the hardware did exactly that, while the simulator
interpreted the instruction as "store the return address in $31 and
then jump to the routine addressed by $31", which resulted in not
taking the jump at all. Simulator corrected.

14-Feb-2011
B_PHYS and B_MAP macros eliminated. They both are only used with the
PDP-11 UNIBUS map functionality.

15-Feb-2011
In the simulated ECO32, if I try to allocate 60 chunks of memory, each
of them having 10000 bytes, I get a kernel panic: exception 22 in kernel
mode. The reason is untested code, executed for the first time ever: the
kernel is doing an "expansion swap" in order to get enough memory for
the process.

16-Feb-2011
Now this is interesting: what is the field "b_bcount" in struct buf
(include/buf.h) good for? Do any transfers with b_bcount != BSIZE
exist? The answer is yes, in two places:
a) swap I/O (src/bio.c)
b) raw I/O (src/bio.c)
(There is a third (mis-)usage of the field b_bcount: it is used as
b_active flag in some block device drivers.)
Raw I/O has to be overhauled in any case, so we concentrate on swap
I/O here. Swap I/O transfers possibly many pages at once, starting
at a given disk block and a given memory address. But this is only
correct under the current "contiguous physical memory per process"
allocation policy. It is useless under any policy which allocates
physical memory for a process non-contiguously - provided that the
transfer mechanism accesses physical memory and not virtual memory
(this is typically the case with DMA). So the kernel will no longer
rely on the disk driver (and possibly the disk hardware) to transfer
multiple blocks in a single call, but split them up into single block
transfers. Done - the "expansion swap" is now running.