x86 binary emulator

x86-16 Binary Register Emulator

When you have updated your program, make sure to Reset / Load first. Then step to perform each line.
You can clear out the current program with the clear button.

Status: Ready

Instruction Monitor

x86 Binary Instruction Cheat Sheet

Type	Mnemonic (Example)	Binary Pattern	Explanation
Prefix	`0x66`	`01100110`	Operand-size override (required for 16-bit regs in 32-bit mode)
MOV	`MOV AX, imm16`	`10111rrr` + lo + hi	Load 16-bit immediate into reg. rrr: AX=000, CX=001, DX=010, BX=011. Little-endian immediate follows.
MOV	`MOV AL, imm8`	`10110rrr` + imm	Load 8-bit immediate. rrr: AL=000, CL=001, DL=010, BL=011, AH=100, CH=101, DH=110, BH=111
ADD	`ADD AL, imm8`	`00000100` + imm	Shortform: add immediate to AL. No ModR/M needed.
ADD	`ADD AX, imm16`	`00000101` + lo + hi	Shortform: add 16-bit immediate to AX. No ModR/M needed.
ADD	`ADD r8, imm8`	`10000000` + ModR/M + imm	ModR/M: `11 000 rrr` — extension field must be 000. rrr = destination register.
ADD	`ADD r16, imm8`	`10000011` + ModR/M + imm	ModR/M: `11 000 rrr`. Sign-extends 8-bit immediate to 16 bits.
ADD	`ADD r/m8, r8`	`00000000` + ModR/M	ModR/M: `11 src dst` — bits 5-3 = source, bits 2-0 = destination.
ADD	`ADD r/m16, r16`	`00000001` + ModR/M	Same ModR/M convention as above. Requires 0x66 prefix for 16-bit operands.

ModR/M Register Encoding

ID (Binary)	8-bit reg	16-bit reg	ModR/M structure (register mode)
000	AL	AX	Bits 7-6: 11 (register-to-register mode) Bits 5-3: source register ID Bits 2-0: destination register ID For `ADD r/m, imm` (0x80/0x83): DDOOOLLL DD: 11 immediate, 00 address no displacement, 01 8-bit displacement, 16-bit displacement OOO: operation 000 for add LLL: register location
001	CL	CX
010	DL	DX
011	BL	BX
100	AH	—
101	CH	—
110	DH	—
111	BH	—

How to construct ADD r/m, r:

Opcode: 00000000 (8-bit) or 00000001 (16-bit)

ModR/M byte: 11 + [Source ID] + [Dest ID]

Example (ADD CL, BL): dst=CL=001, src=BL=011 -> 00000000 11011001

Example (ADD AX, BX): dst=AX=000, src=BX=011 -> 00000001 11011000