EAX is the full 32-bit value; AX is the lower 16-bits; AL is the lower 8 bits; AH is the bits 8 through 15 (zero-based), the top half of AX; So AX is composed of AH:AL halves, and is itself the low half of EAX. (The upper half of EAX isn't directly accessible as a 16-bit register; you can shift or rotate EAX if you want to get at it.)

Lower halves of the 32-bit registers can be used as four 16-bit data registers: AX, BX, CX and DX. Lower and higher halves of the above-mentioned four 16-bit registers can be used as eight 8-bit data registers: AH, AL, BH, BL, CH, CL, DH, and DL.

Fig 3: The 32-bit EAX register retained AX, AH, and AL as sub-registers. The sub-register AX allows us to access the lower 16 bits of EAX. And within AX are the two 8-bit sub-registers, AH and AL.

ax is the 16-bit, "short" size register. It was added in 1979 with the 8086 CPU, but is used in DOS or BIOS code to this day. al and ah are the 8-bit, "char" size registers. al is the low 8 bits, ah is the high 8 bits. They're pretty similar to the old 8-bit registers of the 8008 back in 1972.

Oct 23, 2008 · In the 32-bit days, the 32 bit EAX register was introduced, but the AX, AH, and AL registers were all kept. The designers did not feel it necessary to introduce a new 16 bit register that addressed bits 16 through 31 of EAX.

May 16, 2016 · I'm having trouble understanding registers in x86 Assembly, I know that EAX is the full 32 bits, AX is the lower 16 bits, and then AH and AL the higher and lower 8 bits of AX, But I'm doing a question. If AL=10 and AH=10 what is the value in AX?

This guide describes the basics of 32-bit x86 assembly language programming, covering a small but useful subset of the available instructions and assembler directives. There are several different assembly languages for generating x86 machine code.

Oct 31, 2021 · The least significant byte of AX can be used as a single 8-bit register called AL, while the most significant byte of AX can be used as a single 8-bit register called AH. These names refer to the same physical register.

x86 (technically IA-32) Assembly Overview Registers, Flags, Memory Addressing, Instructions, Stack, Calling Conventions, Directives, Segments BIOS (Basic Input/Output System) + GDB (GNU Debugger) Design Review 3

EAX is a 32-bit register (available only on 386+), ax contains the lower 16 bits (2 bytes) of eax, ah contains the high byte of ax, and al contains the low byte of ax. So ax is 16 bit, al and ah are 8 bit.