Registers instruction: the fourteen 16-bit registers — 8086 assembly guide

The 8086 has fourteen 16-bit registers: four general registers that split into bytes, four pointer and index registers, four segment registers, the instruction pointer and the flags.

Registers are storage inside the CPU, faster than memory and named directly in instructions. Every 8086 register is 16 bits wide. Most instructions can use any general register, but many have a favourite, and some registers are the only ones allowed for a job.

Register	Name	Special roles
AX (AH, AL)	accumulator	MUL, DIV, IN, OUT, LODS, STOS; AH = DOS function
BX (BH, BL)	base	addresses memory ([BX]); the XLAT table
CX (CH, CL)	count	LOOP, REP, JCXZ; CL = shift count
DX (DH, DL)	data	port number for IN/OUT; high half for MUL/DIV
SI	source index	addresses memory; DS:SI for strings
DI	destination index	addresses memory; ES:DI for strings
BP	base pointer	addresses the stack ([BP] uses SS)
SP	stack pointer	top of the stack, SS:SP
CS DS ES SS	segments	code, data, extra, stack
IP	instruction pointer	the next instruction, CS:IP
FLAGS	flags	results of the last operation, CPU control

Halves

AX, BX, CX and DX each split into a high and a low byte: AH and AL make up AX, and so on. They are the same storage, not copies: writing AL changes the low byte of AX, and MOV AX, 0305h sets AH to 3 and AL to 5. SI, DI, BP and SP have no byte halves.

Prints 35.

MOV     AX, 0305h  ; AH = 3, AL = 5
MOV     BX, AX
MOV     AL, BH     ; the high half: 3
ADD     AL, '0'    ; digit -> character
MOV     DL, AL
MOV     AH, 02h    ; DOS: print DL
INT     21h
MOV     AL, BL     ; the low half: 5
ADD     AL, '0'    ; digit -> character
MOV     DL, AL
MOV     AH, 02h    ; DOS: print DL
INT     21h

What you cannot do

Only BX, BP, SI and DI can hold an address in brackets; [AX] or [CX] do not exist. Segment registers take no arithmetic and are loaded only from a general register, memory or the stack. IP is never named: it changes with jumps, calls, returns and interrupts. To read it, call the next line and pop the return address.

CALL    HERE1
HERE1:
POP     BX  ; BX = the offset of HERE1

Conventions

DOS and BIOS calls take their function number in AH and return results in fixed registers, listed for each function. Procedures usually save the registers they change, apart from those that return a result.

Later CPUs

The 80386 widens the general registers to 32 bits (EAX, EBX, ... ESP, with AX still the low half of EAX), widens IP and FLAGS to EIP and EFLAGS, and adds the segment registers FS and GS.