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This is the design of TinyCPU. It's goals are as follows:

1. 8-bit registers and operations (8 bit processor)

2. 16-bit address bus

3. fixed 16-bit instruction length

4. use a small amount of "rich" instructions to do powerful things

5. 1 instruction per clock cycle

Relative moves:

In order to provide uesfulness to the segment-carryover feature, there are a few options for moving a "relative" amount to a register, including IP and SP

A relative move differs in most of the opcodes in that the relative factor is treated as a signed value.

so for instance, a

mov r0,50

mov_relative r0, -10

in the ned, r0 will end up being 40. Although this feature won't see much use in general registers, IP and SP are special because of the option of using the

segment-carryover feature. This means that SP and IP, while being 8-bit registers, can function very similar to a 16-bit register, enabling full usage of the available address space.

Register list:

r0-r5 general purpose registers

sp stack pointer (represented as r6)

ip instruction pointer register (represented as r7)

cs, ds, es, ss segment registers (code segment, data segment, extra segment, stack segment)

tr truth register for conditionals

general opcode format

first byte:

first 4 bits: actual instruction

next 3 bits: (target) register

last 1 bit: conditional

second byte:

first 1 bit: second portion of condition (if not immediate) (1 for only if false)

next 1 bit: use extra segment

next 3 bits: other register. If not 3rd register, top bit specifies which register bank, others unused

last 3 bits: extra opcode information or third register. such as for ADD it could be target=source+third_register

...or second byte is immediate value

For opcodes requiring 3 registers but without room, the target opcode is assume to be the second operation. Such as for AND, target=source AND target

short list of instructions: (not final, still planning)

immediates:

1. move reg, immediate

2. move [reg], immediate

3. push and move reg, immediate (or call immediate)

4. move (relative) reg, immediate

mini-group 5. Root opcode is 5, register is to tell which opcode( up to 8). No register room, only immediate

push immedate

XX

XX

XX

XX

XX

XX

XX

groups: (limited to 2 registers and no immediates. each group has 8 opcodes)

group 1:

move(store) [reg],reg

move(load) reg,[reg]

out reg1,reg2 (output to port reg1 value reg2)

in reg1,reg2 (input from port reg2 and store in reg1)

XX

XX

move segmentreg,reg

move reg,segmentreg

group 2:

and reg1,reg2 (reg1=reg1 and reg2)

or reg, reg

xor reg,reg

not reg1,reg2 (reg1=not reg2)

left shift reg,reg

right shift reg,reg

rotate right reg,reg

rotate left reg,reg

group 3: compares

is greater than reg1,reg2 (TR=reg1>reg2)

is greater or equal to reg,reg

is less than reg,reg

is less than or equal to reg,reg

is equal to reg,reg

is not equal to reg,reg

equals 0 reg

not equals 0 reg

group 4:

push segmentreg

pop segmentreg

push and move reg, reg (or call reg)

exchange reg,reg

exchange reg,seg

XX

XX

group 5:

XX

XX

far jmp reg1, reg2 (CS=reg1 and IP=reg2)

far call reg1,reg2

far jmp [reg] (first byte is CS, second byte is IP)

push extended segmentreg, reg (equivalent to push seg; push reg)

pop extended segmentreg, reg (equivalent to pop reg; pop seg)

reset processor (will completely reset the processor to starting state, but not RAM or anything else)

group 6:

set default register bank to 0 (can be condensed to 1 opcode)

set default register bank to 1

push extended reg, reg

pop extended reg,reg

enable carryover seg

disable carryover seg

mov relative reg, reg

exchange reg, reg

super group: Super groups only have room for 1 register argument. Each subgroup has 8 opcodes, capable of 8 subgroups.

subgroup 0:

push reg

pop reg

set TR

reset TR

increment reg

decrement reg

set register bank 0

set register bank 1

subgroup 1:

enable carryover seg

disable carryover seg

3 register instructions:

1. add reg1, reg2, reg3 (reg1=reg2+reg3)

2. sub reg1, reg2, reg3

opcodes used: 14 of 16. 2 more opcodes available. Decide what to do with the room later.

Possible canidates for opcode compression include

* equals 0 and not equals 0 (room for 7 sub-opcodes each) (not doing that because it'd screw with the easy ALU code

conditionals

1 -- only if true

for only if false, there should basically be another compare or if applicable an always afterwards

limitations that shouldn't be passed with instructions

* Doing 2 memory references

* pushing a memory reference (equates to 2 memory references)

Note it is possible however to read and write 16bits at one time to the memory to consecutive addresses that are 16-bit aligned.

segments:

DS is used in all "normal" memory references

SS is used in all push and pop instructions

ES is used when the ExtraSegment bit is set for either push/pop or normal memory references

CS is only used for fetching instructions

Segment carryover:

In order to overcome the limitations of only having a 256 byte segment, there is a workaround option to "pretend" that IP is a 16 bit register.

When CS carryover is enabled, when IP rollover from 255 to 0 or whatever, CS will be incremented. This makes it so that if you start at address 0:0.

you can continue as far as needed into the address space without having to do ugly far jumps at each of the borders.

Carryover can only be done on CS and SS. The required circuitry is not implemented for DS or ES due to an extreme level of complexity required for it, also

it would only lead to unncessarily complex code

Also of note is that `move relative` implements a "carryover" component. This component will work on either IP or SP, and uses CS and SS respectively.

If used on other registers, there will be no carry over functionality, though it can be used as an easy way to add or subtract an immediate from a register.

States needed:

0. reset

1. decode current instruction (All without memory capable within 1 clock cycle)

2. increment IP(and SP if needed) and fetch next instruction

3. Write 1 register to memory

4. Read 1 register from memory

5. Write 2 registers to memory

6. Read 2 registers from memory

7. Write 1 register to memory and setup increment of sp

8. Write 2 registers to memory and setup double increment of sp

9. Read 1 register from memory and setup decrement of sp

10. Read 2 registers from memory and setup double decrement of sp

11.

registerfile map:

0000: general r0

0001: general r1

0010: general r2

0011: general r3

0100: general r4

0101: general r5

0110: SP (r6)

0111: IP (r7)

1000: second bank r0

1001: second bank r1

1010: second bank r2

1011: second bank r3

1100: CS

1101: DS

1110: ES

1111: SS

Banking works like if(regnumber(2) = '0') then regnumber(3)=regbank; end if;

ALU operations

00000 and reg1,reg2 (reg1=reg1 and reg2)

00001 or reg, reg

00010 xor reg,reg

00011 not reg1,reg2 (reg1=not reg2)

00100 left shift reg,reg (logical)

00101 right shift reg,reg (logical)

00110 rotate right reg,reg

00111 rotate left reg,reg

01000 is greater than reg1,reg2 (TR=reg1>reg2)

01001 is greater or equal to reg,reg

01010 is less than reg,reg

01011 is less than or equal to reg,reg

01100 is equal to reg,reg

01101 is not equal to reg,reg

01110 equals 0 reg

01111 not equals 0 reg

10000 Set TR

10001 Reset TR

10011 Increment

10010 Decrement

10100 Add

10101 Subtract

Alignment restrictions:

In general, their is very few times that a full 16-bit read or 16-bit write is done. These are the times:

* Extended push

* Extended pop

* instruction fetch

Because of this, and because I want for 2 clock cycles to be the longest instruction, I must place some alignment restrictions on the CPU

So, IP must be aligned to a 16-bit address (must be an even number). And SP must also be aligned to a 16-bit address.

Though I don't plan on putting any "real" restriction to setting it to an odd address, nothing will actually work right.

Stack Details:

Because of the need for 16-bit writes and reads of the stack, even though we're usually only using 8-bit values, we end up pushing 2 bytes at one time always.

Stack is oppositely done from the 8086. push X will move X to SS:SP and then increment SP by 2.

Let's take an example program:

--SS is 0

mov sp, 10

push 0xff

after this, 0x00FF will be moved to SS:SP (0x0010) and then sp will be incremented by 2. If we push an 8-bit value, the value is put in the least-significant byte, and the MSB is 0

On Reset:

On reset, all general registers are set to 0

CS is set to 1, IP is set to 0. SS is set to 2 and SP is set to 0.

Carryover is set on CS and not set on SS. DS and ES is 0. TR is false.

Register bank 0 is selected.

Electrical operation:

On power-on, RESET should be high for at least 2 clock cycles. HOLD can optionally be high as well after these two clock cycles.

When HOLD is no longer needed, it should just be turned low and an extra clock cycle should be waited on for it to return to RESET state

When RESET is held low, the processor will execute. It takes 3 clock cycles for the processor to "catch up" to actually executing instructions

Implemented opcode list:

legend:

r = register choice

C = conditional portion

s = segment register choice

i = immediate data

0000 rrrC iiii iiii

mov reg, immediate