Decoding the 8086

INDEX	7 6 5 4 3 2	1 0	7 6	543	210
INSTR	opcode	d w	mod	reg	r/m
EXMPL	1 0 0 0 1 0	0 1	1 1	011	001
ASM	MOV	*	**	BX	CX

* Depending on the order of operands in mov, d will be set to 0 or 1 indicating the direction of the data to be copied, e.g from CX to BX or vice versa. It sets whether reg or r/m is to be considered the destination. e.g. mov cx, bx: cx = dest -> d=0 because CX sits in the r/m field.

** Alternator for r/m. depending on mod the last 3 bits can hold an encoded reference to a register or memory addr

• opcode: e.g. 100010dw or for immediate access 1011wreg
• mod(displacement): alternates between r (register) and m(memory) and enables use of disp_low and disp_high.
  • 00 -> [...]
  • 11 -> r/m = reg
  • 01 -> disp_low [...+8]
  • 10 -> disp_high [...+16]
• d (destination):
  • d=0 -> reg=source and r/m=dest
  • d=1 -> reg=dest and r/m=source (this is required when we want to load data from memory into register as we can only load mem data from r/m field we need to swap dest and source)
• w (wide): w=0 -> 8 bits, w=1 -> 16 bits
• reg: encoded register, e.g. BX
• r/m: encoded register or encoding of memory access, e.g. [BX + 75]

Creating binaries

1. nasm listing_0037_single_register_mov.asm
2. nasm listing_0038_many_register_mov.asm

Assembly on Win

1. nasm -f win64 listing_0037_single_register_mov.asm -o sr_mov.o
2. ld -e sr_mov.o /fo srmov.exe