akkartik
/
mu
1
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity
mu/subx/examples/ex9.subx

50 lines
2.8 KiB
Plaintext
Raw Normal View History

4575 New example, just to fix in my head how arguments go on the stack. It's possible I'm still confused about the order callers push args in to the stack. But even if this violates the calling convention, it should still run.
2018-09-21 22:10:34 +00:00
## Example showing arg order on the stack.
# Show difference between ascii codes of first letter of first arg and first
# letter of second arg.
#
# To run:
# $ subx translate ex9.subx ex9
# $ subx run ex9 z x
# Expected result:
# $ echo $?
# 2
#
# At the start of a SubX program:
# argc: *ESP
# argv[0]: *(ESP+4)
# argv[1]: *(ESP+8)
# ...
# Locals start from ESP-4 downwards.
== code
# instruction effective address operand displacement immediate
# op subop mod rm32 base index scale r32
# 1-3 bytes 3 bits 2 bits 3 bits 3 bits 3 bits 2 bits 2 bits 0/1/2/4 bytes 0/1/2/4 bytes
# s1 = argv[1] (EAX)
8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 0/r32/EAX 8/disp8 . # copy *(ESP+8) to EAX
# s2 = argv[2] (EBX)
8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 3/r32/EBX 0xc/disp8 . # copy *(ESP+12) to EBX
# call string_equal(s1, s2)
# push args
4591
2018-09-22 17:33:41 +00:00
50/push-EAX
53/push-EBX
4575 New example, just to fix in my head how arguments go on the stack. It's possible I'm still confused about the order callers push args in to the stack. But even if this violates the calling convention, it should still run.
2018-09-21 22:10:34 +00:00
# call
e8/call ascii_difference/disp32
# discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add 8 to ESP
# exit(EAX)
89/copy 3/mod/direct 3/rm32/EBX . . . 0/r32/EAX . . # copy EAX to EBX
b8/copy . . . . . . . 1/imm32/exit # copy 1 to EAX
4581 Even more cuddling. We want to keep lines short where the opcode and operands are self-explanatory. If there are any implicit registers, etc., we'll continue to do the table layout. The first two columns look messy now; let's see how this goes. Maybe I'll give up on the tabular layout altogether, just string args with a single space.
2018-09-21 23:55:46 +00:00
cd/syscall 0x80/imm8
4575 New example, just to fix in my head how arguments go on the stack. It's possible I'm still confused about the order callers push args in to the stack. But even if this violates the calling convention, it should still run.
2018-09-21 22:10:34 +00:00
ascii_difference: # (s1, s2) : null-terminated ascii strings
# a = first letter of s1 (ECX)
4590
2018-09-22 17:32:26 +00:00
8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 0/r32/EAX 8/disp8 . # copy *(ESP+8) to EAX
4575 New example, just to fix in my head how arguments go on the stack. It's possible I'm still confused about the order callers push args in to the stack. But even if this violates the calling convention, it should still run.
2018-09-21 22:10:34 +00:00
8b/copy 0/mod/indirect 0/rm32/EAX . . . 0/r32/EAX . . # copy *EAX to EAX
# b = first letter of s2 (EDX)
8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 1/r32/ECX 4/disp8 # copy *(ESP+4) to ECX
8b/copy 0/mod/indirect 1/rm32/ECX . . . 1/r32/ECX . . # copy *ECX to ECX
# a-b
29/subtract 3/mod/direct 0/rm32/EAX . . . 1/r32/ECX . . # subtract ECX from EAX
c3/return