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 serie of processors is that there are few registers to use. This section
describes the main use of each register and ways to use them. That in note that
the rules described here are more suggestions than strict rules. Some operations
need absolutely some kind of registers but most of the you can use any of the freely.
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
EAX EBX ECX EDX
CS DS ES FS GS SS
Index and pointers
ESI EDI EBP EIP ESP
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 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.
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.