0d246473c5
Our infrastructure for displaying errors is far more rudimentary in baremetal. Many ways things can go wrong. But making the attempt seems better than not. I'm also making some effort to keep it easy to see what has been copied over from the top-level, by not modifying copied code to use syntax sugar and so on. It may not be an important enough reason to mix notations in a single file.
49 lines
2.6 KiB
Plaintext
49 lines
2.6 KiB
Plaintext
# Read a single byte from a stream.
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#
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# We need to do this in machine code because streams need to be opaque types,
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# and we don't yet support opaque types in Mu.
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== code
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# instruction effective address register displacement immediate
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# . op subop mod rm32 base index scale r32
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# . 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
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# Return next byte value in eax, with top 3 bytes cleared.
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# Abort on reaching end of stream.
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read-byte: # s: (addr stream byte) -> result/eax: byte
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# . prologue
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55/push-ebp
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89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp
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# . save registers
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51/push-ecx
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56/push-esi
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# esi = s
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 8/disp8 . # copy *(ebp+8) to esi
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# ecx = s->read
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8b/copy 1/mod/*+disp8 6/rm32/esi . . . 1/r32/ecx 4/disp8 . # copy *(esi+4) to ecx
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# if (f->read >= f->write) abort
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3b/compare 0/mod/indirect 6/rm32/esi . . . 1/r32/ecx . . # compare ecx with *esi
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0f 8d/jump-if->= $read-byte:abort/disp32
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# result = f->data[f->read]
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31/xor 3/mod/direct 0/rm32/eax . . . 0/r32/eax . . # clear eax
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8a/copy-byte 1/mod/*+disp8 4/rm32/sib 6/base/esi 1/index/ecx . 0/r32/AL 0xc/disp8 . # copy byte at *(esi+ecx+12) to AL
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# ++f->read
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ff 0/subop/increment 1/mod/*+disp8 6/rm32/esi . . . . 4/disp8 . # increment *(esi+4)
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$read-byte:end:
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# . restore registers
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5e/pop-to-esi
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59/pop-to-ecx
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# . epilogue
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89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp
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5d/pop-to-ebp
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c3/return
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$read-byte:abort:
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(draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 "read-byte: empty stream" 3) # 3=cyan
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{
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eb/jump loop/disp8
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}
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# never gets here
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# . . vim:nowrap:textwidth=0
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