I implemented an 8086 emulator using the 1981 "iAPX 86, 88 USER'S MANUAL". It specifies the opcodes in bit patterns, so add is specified as "0 0 0 0 0 0 d w | mod reg r/m", where d is direction (mem to reg or reg to mem) and w is width (byte or word). Since this kind of pattern is used across many instructions it makes the code fairly easy to comprehend (to me at least).
Extracting the alu function from bits 4-6 means that you can group together the implementation of add, or, adc, sbb, and, sub, xor, to and from memory, for bytes and words into one function.
The code's not as fast as the "one code block per instruction" approach of something like DosBOX but at least it doesn't cause me dread to look at.
Extracting the alu function from bits 4-6 means that you can group together the implementation of add, or, adc, sbb, and, sub, xor, to and from memory, for bytes and words into one function.
The code's not as fast as the "one code block per instruction" approach of something like DosBOX but at least it doesn't cause me dread to look at.