.1 x86.
.1 x86
.1.1 Terminology..
Common for 16-bit (8086/80286), 32-bit (80386, etc.), 64-bit.
byte8-bit. TheDBassemblydirectiveisusedfordefiningvariablesandarraysofbytes. Bytesarepassed
in the 8-bit part of registers:AL/BL/CL/DL/AH/BH/CH/DH/SIL/DIL/R*L.
word16-bit. DW assembly directive —”—. Words are passed in the 16-bit part of the registers:
AX/BX/CX/DX/SI/DI/R*W.
double word(“dword”) 32-bit. DD assembly directive —”—. Double words are passed in registers (x86)
or in the 32-bit part of registers (x64). In 16-bit code, double words are passed in 16-bit register
pairs.
quad word(“qword”)64-bit. DQassemblydirective—”—. In32-bitenvironment, quadwordsarepassed
in 32-bit register pairs.
tbyte(10 bytes) 80-bit or 10 bytes (used for IEEE 754 FPU registers).
paragraph(16 bytes)—term was popular in MS-DOS environment.
Data types of the same width (BYTE, WORD, DWORD) are also the same in WindowsAPI.
.1.2 General purpose registers..
It is possible to access many registers by byte or 16-bit word parts..
It is all inheritance from older Intel CPUs (up to the 8-bit 8080) still supported for backward compatibility.
Older 8-bit CPUs (8080) had 16-bit registers divided by two.
Programs written for 8080 could access the low byte part of 16-bit registers, high byte part or the whole
16-bit register.
Perhaps, this feature was left in 8086 as a helper for easier porting.
This feature is usually not present inRISCCPUs.
Registers prefixed withR-appeared in x86-64, and those prefixed withE-—in 80386.
Thus, R-registers are 64-bit, and E-registers—32-bit.
8 moreGPR’s were added in x86-86: R8-R15..
N.B.: In the Intel manuals the byte parts of these registers are prefixed byL, e.g.:R8L, butIDAnames
these registers by adding theBsuffix, e.g.:R8B.
RAX/EAX/AX/AL
Byte number:
7th 6th 5th 4th 3rd 2nd 1st 0th
RAXx64
EAX
AX
AH ALAKAaccumulator. The result of a function is usually returned via this register.
RBX/EBX/BX/BL
Byte number:
7th 6th 5th 4th 3rd 2nd 1st 0th
RBXx64
EBX
BX
BH BL