## 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&