1 # _write: write to a file descriptor (fd) 2 3 == code 4 # instruction effective address register displacement immediate 5 # . op subop mod rm32 base index scale r32 6 # . 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 7 8 Entry: # just exit; can't test _write just yet 9 # . syscall(exit, 0) 10 bb/copy-to-ebx 0/imm32 11 b8/copy-to-eax 1/imm32/exit 12 cd/syscall 0x80/imm8 13 14 # Since this is the first file of SubX code, a note about type comments. 15 # Eventually we'll build a slightly higher-level safe language atop SubX. 16 # Even though we don't have the safe language yet, we'll start thinking in 17 # terms of the higher-level types in comments. 18 # 19 # Mu will have two kinds of addresses: 20 # - 'ref' which is used to point to a unique element, because machine 21 # code can't store large types in registers. 22 # - 'handle' which can point to a heap allocation, different heap allocations 23 # at different times, or even at times nothing at all. (Later on handles 24 # will turn into fat pointers to enable safe reclamation. But in unsafe 25 # levels we'll just never reclaim them, and handles will be word-sized just 26 # like refs.) 27 # 28 # The type 'address' can be obtained from either a ref or handle, but it can 29 # only be stored on the stack (say to pass objects by reference). 30 # Conversely, a ref can't be copied into another ref, only to an address (which 31 # by construction has a shorter lifetime). 32 # 33 # Beginnings of a lattice of types: 34 # You can convert a ref or handle to an address, but not the other way around. 35 # You can convert addresses to ints, but not the other way around. 36 37 _write: # fd : int, s : (addr array byte) 38 # . prologue 39 55/push-ebp 40 89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp 41 # . save registers 42 50/push-eax 43 51/push-ecx 44 52/push-edx 45 53/push-ebx 46 # syscall(write, fd, (data) s+4, (size) *s) 47 # . fd : ebx 48 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 3/r32/ebx 8/disp8 . # copy *(ebp+8) to ebx 49 # . data : ecx = s+4 50 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 1/r32/ecx 0xc/disp8 . # copy *(ebp+12) to ecx 51 81 0/subop/add 3/mod/direct 1/rm32/ecx . . . . . 4/imm32 # add to ecx 52 # . size : edx = *s 53 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 2/r32/edx 0xc/disp8 . # copy *(ebp+12) to edx 54 8b/copy 0/mod/indirect 2/rm32/edx . . . 2/r32/edx . . # copy *edx to edx 55 # . syscall 56 b8/copy-to-eax 4/imm32/write 57 cd/syscall 0x80/imm8 58 # if (eax < 0) abort 59 3d/compare-eax-with 0/imm32 60 0f 8c/jump-if-lesser $_write:abort/disp32 61 $_write:end: 62 # . restore registers 63 5b/pop-to-ebx 64 5a/pop-to-edx 65 59/pop-to-ecx 66 58/pop-to-eax 67 # . epilogue 68 89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp 69 5d/pop-to-ebp 70 c3/return 71 72 $_write:abort: 73 # can't write a message here for risk of an infinite loop, so we'll use a special exit code instead 74 # . syscall(exit, 255) 75 bb/copy-to-ebx 0xff/imm32 76 b8/copy-to-eax 1/imm32/exit 77 cd/syscall 0x80/imm8 78 # never gets here 79 80 # . . vim:nowrap:textwidth=0