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/spec.tex
87,14 → 87,16
\title{Specification}
\author{Dan Gisselquist, Ph.D.}
\email{dgisselq (at) opencores.org}
\revision{Rev.~0.9}
\revision{Rev.~0.91}
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anchorcolor=black,pdfpagelabels,hypertexnames,
\hypersetup{
ps2pdf,
pdfpagelabels,
hypertexnames,
pdfauthor={Dan Gisselquist},
pdfsubject={Zip CPU}]{hyperref}
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122,6 → 124,7
\begin{revisionhistory}
0.9 & 4/20/2016 & Gisselquist & Modified ISA: LDIHI replaced with MPY, MPYU and MPYS replaced with MPYUHI, and MPYSHI respectively. LOCK instruction now
permits an intermediate ALU operation. \\\hline
0.91& 7/16/2016 & Gisselquist & :escribed three more CC bits\\\hline
0.8 & 1/28/2016 & Gisselquist & Reduced complexity early branching \\\hline
0.7 & 12/22/2015 & Gisselquist & New Instruction Set Architecture \\\hline
0.6 & 11/17/2015 & Gisselquist & Added graphics to illustrate pipeline discussion.\\\hline
521,13 → 524,16
Fig.~\ref{tbl:cc-register},
\begin{table}\begin{center}
\begin{bitlist}
31\ldots 13 & R/W & Reserved for future uses\\\hline
31\ldots 23 & R & Reserved for future uses\\\hline
22\ldots 15 & R/W & Reserved for future uses\\\hline
14 & W & Clear I-Cache command\\\hline
13 & R & VLIW instruction phase (1 for first half)\\\hline
12 & R & (Reserved for) Floating Point Exception\\\hline
11 & R & Division by Zero Exception\\\hline
10 & R & Bus-Error Flag\\\hline
9 & R & Trap, or user interrupt, Flag. Cleared on return to userspace.\\\hline
9 & R & Trap Flag (or user interrupt). Cleared on return to userspace.\\\hline
8 & R & Illegal Instruction Flag\\\hline
7 & R/W & Break--Enable\\\hline
7 & R/W & Break--Enable (sCC), or user break (uCC)\\\hline
6 & R/W & Step\\\hline
5 & R/W & Global Interrupt Enable (GIE)\\\hline
4 & R/W & Sleep. When GIE is also set, the CPU waits for an interrupt.\\\hline
538,7 → 544,7
\end{bitlist}
\caption{Condition Code Register Bit Assignment}\label{tbl:cc-register}
\end{center}\end{table}
the lower 11~bits of the status register form
the lower 15~bits of the status register form
a set of CPU state and condition codes. Writes to other bits of this register
are preserved.
 
586,12 → 592,17
of course.
 
 
The eighth bit is a break enable bit. This controls whether a break
instruction in user mode will halt the processor for an external debugger
(break enabled), or whether the break instruction will simply send send the
CPU into interrupt mode. Encountering a break in supervisor mode will
halt the CPU independent of the break enable bit. This bit can only be set
within supervisor mode.
The eighth bit is a break enable bit. When applied to the supervisor CC
register, this controls whether a break instruction in user mode will halt
the processor for an external debugger (break enabled), or whether the break
instruction will simply send send the CPU into interrupt mode. Encountering
a break in supervisor mode will halt the CPU independent of the break enable
bit. This bit can only be set within supervisor mode. However, when applied
to the user CC register, from supervisor mode, this bit will indicate whether
or not the reason the CPU entered supervisor mode was from a break instruction
or not. This break reason bit is automatically cleared upon any transition to
user mode, although it can also be cleared by the supervisor writing to the
user CC register.
 
% Should break enable be a supervisor mode bit, while the break enable bit
% in user mode is a break has taken place bit?
611,6 → 622,37
supervisor, in supervisor mode, to determine whether it got to supervisor
mode from a trap or from an external interrupt or both.
 
The eleventh bit is a bus error flag. If the user program encountered a bus
error, this bit will be set in the user CC register and the CPU will switch to
supervisor mode. The bit may be cleared by the supervisor, otherwise it is
automatically cleared upon any return to user mode. If the supervisor
encounters a bus error, this bit will be set in the supervisor CC register
and the CPU will halt. In that case, either a CPU reset or a write to the
supervisor CC register will clear this register.
 
The twelth bit is a division by zero exception flag. This operates in a fashion
similar to the bus error flag. If the user attempts to use the divide
instruction with a zero denominator, the system will switch to supervisor mode
and set this bit in the user CC register. The bit is automatically cleared
upon any return to user mode, although it can also be manually cleared by
the supervisor. In a similar fashion, if the supervisor attempts to execute
a divide by zero, the CPU will halt and set the zero exception flag in the
supervisor's CC register. This will automatically be cleared upon any CPU
reset, or it may be manually cleared by the external debugger writing to this
register.
 
The thirteenth bit will operate in a similar fashion to both the bus error
and division by zero flags, only it will be set upon a (yet to be determined)
floating point error.
 
Finally, the fourteenth bit references a clear cache bit. The supervisor may
write a one to this bit in order to clear the CPU instruction cache. The
bit always reads as a zero.
 
Some of the upper bits have been temporarily assigned to indicate CPU
capabilities. This is not a permanent feature, as these upper bits officially
remain reserved.
 
\section{Instruction Format}
All Zip CPU instructions fit in one of the formats shown in
Fig.~\ref{fig:iset-format}.
2464,7 → 2506,7
14\ldots 10 & R/W & Select among one of 32~possible interrupt lines.\\\hline
9\ldots 0 & R/W & Intermediate transfer length minus one. Thus, to transfer
one item at a time set this value to 0. To transfer 1024 at a time,
set it to 1024.\\\hline
set it to 1023.\\\hline
\end{bitlist}
\caption{DMA Control Register Bits}\label{tbl:dmacbits}
\end{center}\end{table}
2478,7 → 2520,7
DMA controller does not use the interrupt controller, and cannot clear
interrupts.) As a third example, if you wished to write to an external
FIFO anytime it was less than half full (had fewer than 512 items), and
interrupt line 2 indicated this condition, you might wish to issue a
interrupt line 3 indicated this condition, you might wish to issue a
\hbox{32'h1fed8dff} to this port.
 
\section{Debug Port Registers}

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