9086/8086_documentation.md

ISA

Instructions vary from 1 to 6 bytes.

Instructions format

6bit	1bit	1bit	2bit	3bit	3bit
Opcode	D bit	W bit	MOD	REG	R/M

• D-bit : the register specified in the Register ID field is a source register (D = 0) or destination register (D =1).
• W-bit : specifies whether the instruction is a byte instruction (W = 0) or a word instruction (W = 1).

On some instructions:

• S-bit : An 8-bit 2's complement number. It can be extended to a 16-bit 2’s complement number depending on the W-bit by making all of the bits in the higher-order byte equal the most significant bit in the low order byte. This is known as sign extension.

S	W	Operation
0	0	8bit operation
0	1	16bit operation with 16bit immediate operand
1	0	invalid?
1	1	16bit operation with a sign extended 8bit immediate operand

• V-bit : V-bit decides the number of shifts for rotate and shift instructions. If V = 0, then count = 1; if V = 1, the count is in CL register. For example, if V = 1 and CL = 2 then shift or rotate instruction shifts or rotates 2-bits
• Z-bit : Used as a compare bit with the zero flag in conditional repeat and loop instructions. ex branch if zero is set or clear.

Register ID / REG	Register Name
0 0 0	AL AX
0 0 1	CL CX
0 1 0	DL DX
0 1 1	BL BX
1 0 0	AH SP
1 0 1	CH BP
1 1 0	DH SI
1 1 1	BH DI

The second byte of the instruction usually identifies the instruction's operands. The MOD (mode) field weather on of the operands is in memory or if both are registers. In some instructions like the immediate-to-memory type the REG field is used as an extension of the opcode. The encoding of R/M depends on how MOD is set. if MOD=11 (register-register mode) then R/M specifies the second Register using the Register ID. otherwise it specifies how the effective address in memory is calculated

R/M	Memory Mode with no displacement [ 0 0 ]	Memory mode with 8 bit displacement [ 0 1 ]	Memory Mode with 16 bit displacement [ 1 0 ]	Register Mode [ 1 1 ] W = 0	Register Mode [ 1 1 ] W = 1
000	[BX] + [SI]	[BX] + [SI] + d8	[BX] + [SI] + d16	AL	AX
001	[BX] + [DI]	[BX] + [DI] + d8	[BX] + [DI] + d16	CL	CX
010	[BP] + [SI]	[BP] + [SI] + d8	[BP] + [SI] + d16	DL	DX
011	[BP] + [DI]	[BP] + [DI] + d8	[BP] + [DI] + d16	BL	BX
100	[SI]	[SI] + d8	[SI] + d16	AH	SP
101	[DI]	[DI] + d8	[DI] + d16	CH	BP
110	d16 (direct)	[BP] + d8	[BP] + d16	DH	SI
111	[BX]	[BX] + d8	[BX] + d16	BH	DI

Example instructions:

Bytecode	AT&T Syntax	meaning
81 c0 aa 55	add $0x55aa,%ax	add 0x55aa to register ax
03 06 aa 55	add 0x55aa,%ax	add the contents of memory locaton 0x55aa to register ax
fe c0	inc %al	increment register al
ff c0	inc %ax	increment register ax
40	inc %ax	increment register ax