Subversion Repositories tinycpu

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

Register list:
r0-r6 general purpose registers
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: unused
next 3 bits: other register
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. push immediate

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)


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




3 register instructions:
1. add reg1, reg2, reg3 (reg1=reg2+reg3)
2. sub reg1, reg2, reg3




0 -nop (doesn't do a thing)
1 -move immediate (only uses first byte)
2 -move
3 -push
4 -push immediate
5 -push and move (or call when acting on ip)
6 -compare (is less than, is less than or equal, is greater than, is greater than or equal, is equal, is not equal) (6 conditions room for 2 more in extra)
7 -add
8 -subtract
9 -bitwise operations (xor, or, and, shift right, shift left, not)

x -multiply (if room)
x -divide


conditionals
0 -- always
1 -- only if true
for only if false, there should basically be another compare or if applicable an always afterwards

push
pop
move
add
sub