Getting Started

You're going to be learning assembler.

Know your environment

.COM file defaults

Knowing what register values are initialized at program start can save you the trouble of having to set them in your code. On most (but not all) DOS environments, the following registers have these default values:

AX=0000 BX=0000 CX=00FF DX=Same as CS register SI=0100 DI=FFFE SP=FFFC (DOS) or FFFE (Windows)

The rest can't be counted on for any specific value, except that BP is usually 09xx so you can usually count on the high byte being 09h

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	80386+	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 words [SI] - ES:[DI], advance SI,DI
A6	CMPSB		Compare bytes DS:[SI] - ES:[DI], advance SI,DI
A7	CMPSD	80386+	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	80386+	Sign-extend word AX to doubleword EAX
27	DAA		Decimal adjust AL after addition
2F	DAS		Decimal adjust AL after subtraction
F4	HLT		Halt	Resumes operation if an interrupt occurs; could use this for pacing effects that run too fast
EC	IN AL,DX		Input byte from port DX into AL
ED	IN eAX,DX		Input word from port DX into eAX
6C	INS rmb,DX	80186+	Input byte from port DX into [DI], advance DI
6D	INS rmv,DX	80186+	Input word from port DX into [DI], advance DI
6C	INSB	80186+	Input byte from port DX into ES:[DI], advance DI
6D	INSD	80386+	Input dword from port DX into ES:[DI], advance DI
6D	INSW	80186+	Input word from port DX into ES:[DI], advance DI
CC	INT 3		Interrupt 3 (trap to debugger)	If performing very many CALLs to a single procedure, could make it INT 3
CE	INTO		Interrupt 4 if overflow flag is 1
CF	IRET		Interrupt return (far return and pop flags)
CF	IRETD	80386+	Interrupt return (pop EIP, ECS, Eflags)
9F	LAHF		Load: AH = flags SF ZF xx AF xx PF xx CF
C9	LEAVE	80186+	Set SP to BP, then POP BP (reverses previous ENTER)
AC	LODS mb		Load byte [SI] into AL, advance SI
AD	LODS mv		Load word [SI] into eAX, advance SI
AC	LODSB		Load byte [SI] into AL, advance SI
AD	LODSD	80386+	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 word [SI] to ES:[DI], advance SI,DI
A4	MOVSB		Move byte DS:[SI] to ES:[DI], advance SI,DI
A5	MOVSD	80386+	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	80186+	Output byte [SI] to port number DX, advance SI
6F	OUTS DX,rmv	80186+	Output word [SI] to port number DX, advance SI
6E	OUTSB	80186+	Output byte DS:[SI] to port number DX, advance SI
6F	OUTSD	80386+	Output dword DS:[SI] to port number DX, advance SI
6F	OUTSW	80186+	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	80186+	Pop DI,SI,BP,x ,BX,DX,CX,AX (SP value is ignored)
61	POPAD	80386+	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	80386+	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	80186+	Push AX,CX,DX,BX,original SP,BP,SI,DI
60	PUSHAD	80386+	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	80386+	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 words eAX - ES:[DI], advance DI
AE	SCASB		Compare bytes AL - ES:[DI], advance DI
AF	SCASD	80386+	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	80386+	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]

Where to go from here?

Tips, Tricks, and Techniques can help you with ideas on optimizing your next production, or help you design while you're writing it.

Some Case Studies are provided that illustrate and explain some of the choices made when sizecoding.

Can't find what you need? Check our list of external resources.