S2 (a 32-bit version of S1)

description
instruction format
opcode encoding
meaning

Description

S2 is a 32-bit version of S1, in other words, it is a S1 on steroid. The main aim of of S2 design is to increase the address space. The address space of S1 is 10 bits hence 1024 words which is not enough to run any realistic program. There are enough bit in an instruction to encode three addresses, therefore 3-address instruction format is adopted. The instruction set is more "complete" such as full integer arithmetic and logic to enable compiling a realistic program into this ISA.

Three new addressing modes are introduced: displacement, index and immediate. These mode enables a more realistic access to data structure such as a local variable in an activation record and array indexing. To sum up, S2 ISA represents a realistic ISA for a typical 32-bit processor.

This is not "optimal" design. There are plenty of room for improvement.

Instruction format

L-format    op:5 rd1:5 ads:22
D-format    op:5 rd1:5 rs2:5 disp:17
X-format    op:5 rd1:5 rs2:5 rs3:5 xop:12
(rd dest, rs source, ads and disp sign extended)

L-format is similar to S1. D-format is new. X-format resembles S-format of S1. Instructions are fixed length at 32-bit. Register set is 32, with R[0] always return zero. The address space is 32-bit, addressing is word. Flags are: Z zero, S sign, C carry, O overflow/underflow.

ISA and opcode encoding

mode: a - absolute, d -displacement, x - index, i - immediate,
r - register, r2 - register 2 operands, s - special 1 operand

opcode op mode format
0 ld     a     L
1 ld     d     D
2 ld     i     L
3 st     a     L
4 st     d     D
5 jmp    a     L use r1 as condition
6 jal    a     L
7 add    i     D
8 sub    i     D
9 mul    i     D
10 div   i     D
11 and   i     D
12 or    i     D
13 xor   i     D
14..30 undefined
31 xop   -     X

jump conditional coding in r1 field:
0 always, 1 eq, 2 neq, 3 lt, 4 le, 5 ge, 6 gt

xop
0 add    r     X
1 sub    r     X
2 mul    r     X
3 div    r     X
4 and    r     X
5 or     r     X
6 xor    r     X
7 shl    r2    X
8 shr    r2    X
9 ld     x     X
10 st    x     X
11 jr    s     X use r1
12 trap s     X use r1 as number of trap
13..4095 undefined

Meaning

format: op dest, source1, source2
r0 always returns zero

ld r1,ads         R[r1] = M[ads]
ld r1,#n          R[r1] = n
ld r1,d(r2)       R[r1] = M[ d + R[r2] ]
ld r1,(r2+r3)     R[r1] = M[ R[r2] + R[r3] ]
st ads,r1         M[ads] = R[r1]
st d(r2),r1       M[ d + R[r2] ] = R[r1]
st (r2+r3),r1     M[ R[r2] + R[r3] ] = R[r1]
jmp cond,ads      if cond true PC = ads
    cond: always, eq Z=0, neq Z=1, lt S=1,
    le S=1 or Z=1, ge S=0 or Z=1, gt S=0

jal r1,ads R[r1] = PC; PC = ads ;; jump and link, to subroutine
jr r1 PC = R[r1] ;; return from subroutine

arithmetic: two-complement integer arithmetic

add r1,r2,r3 R[r1] = R[r2] + R[r3]
add r1,r2,#n R[r1] = R[r2] + sign extended n (n is 17-bit)
...

add, sub affect Z,C -- C indicates carry (add) or borrow(sub)
mul, div affect Z,O -- O indicates overlow (mul) or underflow (div) and divide by zero

logic (bitwise)
and r1,r2,r3     R[r1] = R[r2] bitand R[r3]
and r1,r2,#n     R[r1] = R[r2] bitand sign extended n
or xor . . .
shl r1,r2        R[r1] = R[r2] shift left one bit
shr r1,r2        R[r1] = R[r2] shift right one bit

affect Z,S bits

trap n r2 special instruction, n is in r1-field.

The opcode format and assembly language format for S2 follow the tradition dest = source1 op source2 from PDP, VAX and IBM S360. As r0 always is zero, many instructions can be synthesis using r0.

or r1,r2,r0         move r1 <- r2
or r1,r0,r0         clear r1
sub r0,r1,r2        compare r1 r2 affects flags

To complement a register, xor with 0xFFFFFFFF (-1) can be used.

xor r1,r2,#-1 r1 = complement r2

28 November 2001
end