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Slide 3.10: Instruction operands Slide 3.12: Integer constants Home         Print version

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General-Purpose Registers

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EAX, EBX, ECX, and EDX.

EAX Register Output

EAX is AX, which can be further addressed as two separate 8-bit values: AH and AL.

AH AL   AX EAX

BH BL   BX EBX

CH CL   CX ECX

DH DL   DX EDX

• EAX (Extended Accumulator Register), the primary accumulator, is used for operations involving input/output and most arithmetic. EAX is automatically used by multiplication and division instructions.
• EBX (Extended Base Register) is known as the base register since it is used as an index to extend addressing.
• ECX (Extended Count Register) is know as the count register. It may contain a value to control the number of times a loop is repeated or a value to shift bits left or right. ECX is used as a loop counter.
• EDX (Extended Data Register) is known as the data register. Some input/output operations require its use, and multiply and divide operations that involve large values.