https://github.com/akkartik/mu/blob/main/112read-byte.subx
  1 # Read a single byte from a stream.
  2 #
  3 # We need to do this in machine code because streams need to be opaque types,
  4 # and we don't yet support opaque types in Mu.
  5 
  6 == code
  7 #   instruction                     effective address                                                   register    displacement    immediate
  8 # . op          subop               mod             rm32          base        index         scale       r32
  9 # . 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
 10 
 11 # Return next byte value in eax, with top 3 bytes cleared.
 12 # Abort on reaching end of stream.
 13 read-byte:  # s: (addr stream byte) -> result/eax: byte
 14     # . prologue
 15     55/push-ebp
 16     89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
 17     # . save registers
 18     51/push-ecx
 19     56/push-esi
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