Getting Started

You're going to be learning assembler.

Know your environment

"default environment settings"

1-byte opcodes

The 80x86 family was originally a CISC design, which is a design philosophy that intentionally attempts to create many instructions that perform multiple steps. In sizecoding, you are trying to perform as much work in as little space as possible, so it is helpful to know (or memorize!) every 1-byte instruction in the 80x86 family. Here's a handy chart (segments and prefixes omitted):

Opcode	Mnemonic	Arch	Description	Notes
37	AAA		ASCII adjust AL (carry into AH) after addition
3F	AAS		ASCII adjust AL (borrow from AH) after subtraction
98	CBW		Convert byte into word (AH = top bit of AL)
99	CDQ	3	Convert dword to qword (EDX = top bit of EAX)
F8	CLC		Clear carry flag
FC	CLD		Clear direction flag so SI and DI will increment
FA	CLI		Clear interrupt enable flag; interrupts disabled
F5	CMC		Complement carry flag
A6	CMPS mb,mb		Compare bytes [SI] - ES:[DI], advance SI,DI
A7	CMPS mv,mv		Compare vwords [SI] - ES:[DI], advance SI,DI
A6	CMPSB		Compare bytes DS:[SI] - ES:[DI], advance SI,DI
A7	CMPSD	3	Compare dwords DS:[SI] - ES:[DI], advance SI,DI
A7	CMPSW		Compare words DS:[SI] - ES:[DI], advance SI,DI
99	CWD		Convert word to doubleword (DX = top bit of AX)
98	CWDE	3	Sign-extend word AX to doubleword EAX
27	DAA		Decimal adjust AL after addition
2F	DAS		Decimal adjust AL after subtraction
F4	HLT		Halt
EC	IN AL,DX		Input byte from port DX into AL
ED	IN eAX,DX		Input vword from port DX into eAX
6C	INS rmb,DX	1	Input byte from port DX into [DI], advance DI
6D	INS rmv,DX	1	Input vword from port DX into [DI], advance DI
6C	INSB	1	Input byte from port DX into ES:[DI], advance DI
6D	INSD	3	Input dword from port DX into ES:[DI], advance DI
6D	INSW	1	Input vword from port DX into ES:[DI], advance DI
CC	INT 3		Interrupt 3 (trap to debugger)
CE	INTO		Interrupt 4 if overflow flag is 1
CF	IRET		Interrupt return (far return and pop flags)
CF	IRETD	3	Interrupt return (pop EIP, ECS, Eflags)
9F	LAHF		Load: AH = flags SF ZF xx AF xx PF xx CF
C9	LEAVE	1	Set SP to BP, then POP BP (reverses previous ENTER)
AC	LODS mb		Load byte [SI] into AL, advance SI
AD	LODS mv		Load vword [SI] into eAX, advance SI
AC	LODSB		Load byte [SI] into AL, advance SI
AD	LODSD	3	Load dword [SI] into EAX, advance SI
AD	LODSW		Load word [SI] into AX, advance SI
A4	MOVS mb,mb		Move byte [SI] to ES:[DI], advance SI,DI
A5	MOVS mv,mv		Move vword [SI] to ES:[DI], advance SI,DI
A4	MOVSB		Move byte DS:[SI] to ES:[DI], advance SI,DI
A5	MOVSD	3	Move dword DS:[SI] to ES:[DI], advance SI,DI
A5	MOVSW		Move word DS:[SI] to ES:[DI], advance SI,DI
90	NOP		No Operation
EE	OUT DX,AL		Output byte AL to port number DX
EF	OUT DX,eAX		Output word eAX to port number DX
6E	OUTS DX,rmb	1	Output byte [SI] to port number DX, advance SI
6F	OUTS DX,rmv	1	Output word [SI] to port number DX, advance SI
6E	OUTSB	1	Output byte DS:[SI] to port number DX, advance SI
6F	OUTSD	3	Output dword DS:[SI] to port number DX, advance SI
6F	OUTSW	1	Output word DS:[SI] to port number DX, advance SI
1F	POP DS		Set DS to top of stack, increment SP by 2
07	POP ES		Set ES to top of stack, increment SP by 2
17	POP SS		Set SS to top of stack, increment SP by 2
61	POPA	1	Pop DI,SI,BP,x ,BX,DX,CX,AX (SP value is ignored)
61	POPAD	3	Pop EDI,ESI,EBP,x,EBX,EDX,ECX,EAX (ESP ign.)
9D	POPF		Set flags register to top of stack, increment SP by 2
9D	POPFD	3	Set eflags reg to top of stack, incr SP by 2
0E	PUSH CS		Set [SP-2] to CS, then decrement SP by 2
1E	PUSH DS		Set [SP-2] to DS, then decrement SP by 2
06	PUSH ES		Set [SP-2] to ES, then decrement SP by 2
16	PUSH SS		Set [SP-2] to SS, then decrement SP by 2
60	PUSHA	1	Push AX,CX,DX,BX,original SP,BP,SI,DI
60	PUSHAD	3	Push EAX,ECX,EDX,EBX,original ESP,EBP,ESI,EDI
9C	PUSHF		Set [SP-2] to flags register, then decrement SP by 2
9C	PUSHFD	3	Set [SP-4] to eflags reg, then decr SP by 4
C3	RET		Return to caller (near or far, depending on PROC)
CB	RETF		Return to far caller (pop offset, then seg)
C3	RETN		Return to near caller (pop offset only)
9E	SAHF		Store AH into flags SF ZF xx AF xx PF xx CF
AE	SCAS mb		Compare bytes AL - ES:[DI], advance DI
AF	SCAS mv		Compare vwords eAX - ES:[DI], advance DI
AE	SCASB		Compare bytes AL - ES:[DI], advance DI
AF	SCASD	3	Compare dwords EAX - ES:[DI], advance DI
AF	SCASW		Compare words AX - ES:[DI], advance DI
36	SS		Use SS segment for the following memory reference
F9	STC		Set carry flag
FD	STD		Set direction flag so SI and DI will decrement
FB	STI		Set interrupt enable flag, interrupts enabled
AA	STOS mb		Store AL to byte [DI], advance DI
AB	STOS mv		Store eAX to word [DI], advance DI
AA	STOSB		Store AL to byte ES:[DI], advance DI
AB	STOSD	3	Store EAX to dword ES:[DI], advance DI
AB	STOSW		Store AX to word ES:[DI], advance DI
9B	WAIT		Wait until floating-point operation is completed
D7	XLAT		Set AL to memory byte DS:[BX + unsigned AL]

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