x86 assembly multiply and divide instruction operands, 16-bit and higher

Question

I\'m rather confused about how the multiply and divide operations work in x86 assembly. For example, the code below doesn\'t seem too difficult since deals with 8-bit.

8

Answer 1

A quick glance at the documentation shows that there are 4 possible operand sizes for MUL. The inputs and outputs are summarized in a handy table:

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| Operand Size | Source 1 | Source 2   | Destination |
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| Byte         | AL       | r/m8       | AX          |
| Word         | AX       | r/m16      | DX:AX       |
| Doubleword   | EAX      | r/m32      | EDX:EAX     |
| Quadword     | RAX      | r/m64      | RDX:RAX     |
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