EAX
It’s called the accumulator register because it’s the primary register used for common calculations (such as ADD and SUB). While other registers can be used for calculations, EAX has been given preferential status by assigning it more efficient, one-byte opcodes.
AX
The least significant 16 bits of EAX which can be further broken down into AH (the 8 most significant bits of AX) and AL (the 8 least significant bits).
AH
The 8 most significant bits of AX.
AL
The 8 least significant bits of AX.
EBX
The Base Register. In 32-bit architecture, EBX doesn’t really have a special purpose so just think of it as a catch-all for available storage. Like EAX, it can be referenced in whole (EBX) or in part (BX, BH, BL).
ECX
The Counter Register. As its name implies, the counter (or count) register is frequently used as a loop and function repetition counter, though it can also be used to store any data. Like EAX, it can be referenced in whole (ECX) or in part (CX, CH, CL).
EDX
The Data Register. EDX is kind of like a partner register to EAX. It’s often used in mathematical operations like division and multiplication to deal with overflow where the most significant bits would be stored in EDX and the least significant in EAX. It is also commonly used for storing function variables. Like EAX, it can be referenced in whole (EDX) or in part (DX, DH, DL).
ESI
The Source Index. The counterpart to EDI, ESI is often used to store the pointer to a read location. For example, if a function is designed to read a string, ESI would hold the pointer to the location of that string.
EDI
The Destination Index. Though it can be (and is) used for general data storage, EDI was primarily designed to store the storage pointers of functions, such as the write address of a string operation.
EBP
The Base Pointer. EBP is used to keep track of the base/bottom of the stack. It is often used to reference variables located on the stack by using an offset to the current value of EBP, though if parameters are only referenced by register, you may choose to use EBP for general use purposes.
ESP
The Stack Pointer. ESP is used to track the top of the stack. As items are moved to and from the stack ESP increments/decrements accordingly. Of all of the general purpose registers, ESP is rarely/never used for anything other than it’s intended purpose.
EIP
The Instruction Pointer. Not a general purpose register, but fitting to cover here, EIP points to the memory address of the next instruction to be executed by the CPU.
EFLAGS
The EFLAGS register is comprised of a series of flags that represent Boolean values resulting from calculations and comparisons and can be used to determine when/if to take conditional jumps
[EBX]
Refers to the value stored at the memory address in EBX. When you see a value in brackets such as ADD DWORD PTR [X] or MOV eax, [ebx] it is referring to the value stored at memory address X. Relevant size keywords: BYTE = 1 byte, WORD = 2 bytes, DWORD = 4 bytes .
SI/DI/SP/BP
The least significant 16 bits of ESI/EDI/ESP/EBP. Unlike the X-registers these can’t be broken down into 8-bit segments.
CS
16-bit code segment register
DS
16-bit data segment register
SS
16-bit stack segment register
ES/FS/GS
16-bit data segment registers
ST0 to ST7
80-bit floating point registers.
Extensions
Intel-specific features that use the MMX and XMM registers. MMX, SSE, SSE2, SSE3, etc.
MMX
64-bit registers (mm0 to MM7) carved out of the 80-bit ST0 to ST7 floating point registers. Lower 64 bits of the ST registers.
XMM
128-bit registers (XMM0 through XMM7).
