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## example showing file syscalls
# Create a file, open it for writing, write a character to it, close it, open
# it for reading, read a character from it, close it, delete it, and return
# the character read.
#
# To run:
# $ subx translate ex8.subx ex8
# $ subx run ex8
# Expected result:
# $ echo $?
# 97
== 0x08048074 # code segment, after leaving room for ELF header and segment headers
# 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
## creat(filename)
bb/copy . . . . . . . 0x08049131/imm32/fname # copy to EBX
b9/copy . . . . . . . 0x180/imm32/fixed-perms # copy 0 to ECX
b8/copy . . . . . . . 8/imm32/creat # copy 8 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h
## fd = open(filename, O_WRONLY, 0)
bb/copy . . . . . . . 0x08049131/imm32/fname # copy to EBX
b9/copy . . . . . . . 1/imm32/wronly # copy 1 to ECX
ba/copy . . . . . . . 0x180/imm32/fixed-perms # copy 0 to EDX
b8/copy . . . . . . . 5/imm32/open # copy 5 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h
# save fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
89/copy 0/mod/indirect 3/rm32/EBX 0/r32/EAX # copy EAX to *EBX
## write(fd, "a", 1)
# load fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
8b/copy 0/mod/indirect 3/rm32/EBX 3/r32/EBX # copy *EBX to EBX
#
b9/copy . . . . . . . 0x08049129/imm32/a # copy to ECX
ba/copy . . . . . . . 1/imm32/size # copy 1 to EDX
b8/copy . . . . . . . 4/imm32/write # copy 4 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h
## close(fd)
# load fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
8b/copy 0/mod/indirect 3/rm32/EBX 3/r32/EBX # copy *EBX to EBX
#
b8/copy . . . . . . . 6/imm32/close # copy 6 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h
## fd = open(filename, O_RDONLY, 0)
bb/copy . . . . . . . 0x08049131/imm32/fname # copy to EBX
b9/copy . . . . . . . 0/imm32/rdonly # copy 0 to ECX
ba/copy . . . . . . . 0x180/imm32/fixed-perms # copy 0 to EDX
b8/copy . . . . . . . 5/imm32/open # copy 5 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h
# save fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
89/copy 0/mod/indirect 3/rm32/EBX 0/r32/EAX # copy EAX to *EBX
## read(fd, b, 1)
# load fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
8b/copy 0/mod/indirect 3/rm32/EBX 3/r32/EBX # copy *EBX to EBX
#
b9/copy . . . . . . . 0x0804912d/imm32/b # copy to ECX
ba/copy . . . . . . . 1/imm32/size # copy 1 to EDX
b8/copy . . . . . . . 3/imm32/read # copy 3 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h
## close(fd)
# load fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
8b/copy 0/mod/indirect 3/rm32/EBX 3/r32/EBX # copy *EBX to EBX
#
b8/copy . . . . . . . 6/imm32/close # copy 8 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h
## unlink(filename)
bb/copy . . . . . . . 0x08049131/imm32/fname # copy to EBX
b8/copy . . . . . . . 0xa/imm32/unlink # copy 8 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h
## exit(b)
# load b
bb/copy . . . . . . . 0x0804912d/imm32/b # copy to EBX
8b/copy 0/mod/indirect 3/rm32/EBX 3/r32/EBX # copy *EBX to EBX
#
b8/copy . . . . . . . 1/imm32/exit # copy 1 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h
== 0x08049125 # data segment
00 00 00 00 # fd
61 00 00 00 # a: string to write to file: 'a'
00 00 00 00 # b: space for string read from file
2e 66 6f 6f 00 00 00 00 # filename: '.foo'
# vim:ft=subx:nowrap:tw&
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