Registers & Computer Memory Primer for x86

Architecture Basics

Registers & Computer Memory Primer for x86


The main tools to write programs in x86 assembly are the processor registers. The registers are like variables built in the processor. Using registers instead of memory to store values makes the process faster and cleaner. The problem with the x86 series of processors is that there are few registers to use.

Modern x86 processors have eight 32-bit general-purpose registers, as depicted in the figure below. The register names are mostly historical. For example, EAX used to be called the accumulator since it was used by a number of arithmetic operations, and ECX was known as the counter since it was used to hold a loop index. Whereas most of the registers have lost their special purposes in the modern instruction set, by convention, two are reserved for special purposes — the stack pointer (ESP) and the base pointer (EBP). More details are given below.

Here is a list of the available registers on the 386 and higher processors. This list shows the 32 bit registers. Most of the can be broken down to 16 or even 8 bits registers.

General registers

Segment registers

Index and pointers


General registers
As the title says, general register are the one we use most of the time Most of the instructions perform on these registers. They all can be broken down into 16 and 8 bit registers.

32 bits :  EAX EBX ECX EDX
16 bits : AX BX CX DX
 8 bits : AH AL BH BL CH CL DH DL

The “H” and “L” suffix on the 8 bit registers stand for high byte and low byte. With this out of the way, let’s see their individual main use

EAX,AX,AH,AL : Called the Accumulator register. 
               It is used for I/O port access, arithmetic, interrupt calls,

EBX,BX,BH,BL : Called the Base register
               It is used as a base pointer for memory access
               Gets some interrupt return values

ECX,CX,CH,CL : Called the Counter register
               It is used as a loop counter and for shifts
               Gets some interrupt values

EDX,DX,DH,DL : Called the Data register
               It is used for I/O port access, arithmetic, some interrupt 

Segment registers

Segment registers hold the segment address of various items. They are only available in 16 values. They can only be set by a general register or special instructions. Some of them are critical for the good execution of the program and you might want to consider playing with them when you’ll be ready for multi-segment programming

CS         : Holds the Code segment in which your program runs.
             Changing its value might make the computer hang.

DS         : Holds the Data segment that your program accesses.
             Changing its value might give erronous data.

ES,FS,GS   : These are extra segment registers available for
             far pointer addressing like video memory and such.

SS         : Holds the Stack segment your program uses.
             Sometimes has the same value as DS.
             Changing its value can give unpredictable results,
             mostly data related.

Indexes and pointers

Indexes and pointer and the offset part of and address. They have various uses but each register has a specific function. They some time used with a segment register to point to far address (in a 1Mb range). The register with an “E” prefix can only be used in protected mode.

ES:EDI EDI DI : Destination index register
                Used for string, memory array copying and setting and
                for far pointer addressing with ES

DS:ESI EDI SI : Source index register
                Used for string and memory array copying

SS:EBP EBP BP : Stack Base pointer register
                Holds the base address of the stack
SS:ESP ESP SP : Stack pointer register
                Holds the top address of the stack

CS:EIP EIP IP : Index Pointer
                Holds the offset of the next instruction
                It can only be read 

The EFLAGS register

The EFLAGS register hold the state of the processor. It is modified by many intructions and is used for comparing some parameters, conditional loops and conditionnal jumps. Each bit holds the state of specific parameter of the last instruction. Here is a listing :

Bit   Label    Desciption
0      CF      Carry flag
2      PF      Parity flag
4      AF      Auxiliary carry flag
6      ZF      Zero flag
7      SF      Sign flag
8      TF      Trap flag
9      IF      Interrupt enable flag
10     DF      Direction flag
11     OF      Overflow flag
12-13  IOPL    I/O Priviledge level
14     NT      Nested task flag
16     RF      Resume flag
17     VM      Virtual 8086 mode flag
18     AC      Alignment check flag (486+)
19     VIF     Virutal interrupt flag
20     VIP     Virtual interrupt pending flag
21     ID      ID flag

Those that are not listed are reserved by Intel.

Undocumented registers

There are registers on the 80386 and higher processors that are not well documented by Intel. These are divided in control registers, debug registers, test registers and protected mode segmentation registers. As far as I know, the control registers, along with the segmentation registers, are used in protected mode programming, all of these registers are available on 80386 and higher processors except the test registers that have been removed on the pentium. Control registers are CR0 to CR4, Debug registers are DR0 to DR7, test registers are TR3 to TR7 and the protected mode segmentation registers are GDTR (Global Descriptor Table Register), IDTR (Interrupt Descriptor Table Register), LDTR (Local DTR), and TR.

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