MiniISA has four registers: $0, $1, $2 and $r, represented as 00, 01, 10 and 11
respectively. $r stands for "result", and is as such the register into which
the results of math operations are placed. $2 is customarily the stack pointer.
The special register $01 is actually the 16-bit representation of registers $0
and $1, with $0 as the high bits.
_'s in the bit representation of operations are ignored bits.
bits name function
----------------------------------------
0000aa__ load $a load into register a from address in $01
0001aa__ store $a store from register a into address in $01
0010____ jump jump to address in $01
0011oooo jump o jump o instructions (signed)
0100iiii high $0 i high bits of register 0 set to iiii
0101iiii low $0 i low bits of 0
0110iiii high $1 i high bits of 1
0111iiii low $1 i low bits of 1
1000aabb sub $a $b $r = $a - $b
1001oooo branch o if ($r < 0) jump o instructions (signed)
1010aabb swap $a $b $a and $b are swapped
1100aabb srl $r = $a >> $b (logical)
1101aabb sra $r = $a >> $b (arithmetic)
111_aabb sll $r = $a << $b
jumps:
A jump to a given address will actually continue execution at the immediate
next address. That is, the program counter is always incremented, even on
jumps. For this reason, an assembler implementing labels should make jumps go
to the address immediately preceeding a given label.
pseudoinstructions:
imm $a 0xAB
high $a 0xA
low $a 0xB
addr 0xABCD
high $0 0xA
low $0 0xB
high $1 0xC
low $1 0xD
jump <label>
(if the label is within 8 instructions)
jump <offset>
(otherwise)
addr <label>
jump
branch <label>
(if the label is within 8 instructions)
branch <offset>
(otherwise)
branch 1
jump 5
addr <label>
jump