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## read a character from stdin, save it to a local on the stack, write it to stdout
== 0x08048054 # code segment, after leaving room for ELF header and segment headers
# opcode ModR/M SIB displacement immediate
# instruction mod, reg, Reg/Mem bits scale, index, base
# 1-3 bytes 0/1 byte 0/1 byte 0/1/2/4 bytes 0/1/2/4 bytes
## function main
# prolog
55 # push EBP
89 e5 # copy ESP to EBP
# ModR/M: 11 (direct mode) 100 (src ESP) 101 (dest EBP)
# allocate x on the stack
81 ec 4/imm32 # subtract 4 bytes from ESP
# ModR/M: 11 (direct mode) 101 (subtract imm32) 100 (dest EBP)
## read(stdin, x, 1)
# fd = 0 (stdin)
bb 0/imm32 # copy 0 to EBX
# set location to read character to
89 e9 # copy EBP to ECX
# ModR/M: 11 (direct mode) 101 (src EBP) 001 (dest ECX)
# size = 1 character
ba 1/imm32 # copy 1 to EDX
# syscall = read
b8 3/imm32 # copy 3 to EAX
# call
cd 128/imm8 # int 80h
## write(stdout, x, 1)
# fd = 1 (stdout)
bb 1/imm32 # copy 1 to EBX
# set location of character to write out
89 e9 # copy EBP to ECX
# ModR/M: 11 (direct mode) 101 (src EBP) 001 (dest ECX)
# size = 1 character
ba 1/imm32 # copy 1 to EDX
# syscall = write
b8 4/imm32 # copy 4 to EAX
# call
cd 128/imm8 # int 80h
## exit(EBX)
b8 1/imm32 # copy 1 to EAX
cd 128/imm8 # int 80h
# vim:ft=subx:nowrap
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