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0xAX merged 1 commit into from
Mar 14, 2026
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asm-4: add missing instructions #81

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15 changes: 12 additions & 3 deletions content/asm_4.md
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Expand Up @@ -63,9 +63,18 @@ cmovne rax, rdx

We use the decimal arithmetic instructions to operate with integer numbers. We have already seen these instructions in one of the previous chapters:

- `add` - Addition. It adds the value of the second operand to the first and stores the result in the first operand.
- `sub` - Subtraction. It subtracts the value of the second operand from the first and stores the result in the first operand.
- `div` and `idiv` - Unsigned and signed division. Both instructions take a single operand. The value of this operand is divided by the value of the `rax` register. The place where the result is stored depends on the size of the operands. In the case of 8-bit operands, the result is stored in the `al:ah` pair. In the case of 16-bit operands, the result is stored in the `dx:ax` pair. For 32-bit operands, the result is stored in the `edx:eax`, and in the case of 64-bit operands, the result is stored in the `rdx:rax` pair.
- `add` - Addition. It adds the value of the second operand to the first and stores the result in the first operand.
- `sub` - Subtraction. It subtracts the value of the second operand from the first and stores the result in the first operand.
- `mul` and `imul` - Unsigned and signed multiplication. Both instructions take a single operand. It multiplies the value of the single operand by the value stored in the `rax` register or its part. The part where the result is stored depends on the size of the operands.
- For 8-bit operands, the value from the `al` register is multiplied by the operand, and the result is stored in the `ax` register.
- For 16-bit operands, the value of the `ax` register is multiplied, and the result is stored in the `dx:ax` register pair.
- For 32-bit operands, the value of the `eax` register is multiplied, and the result is stored in the `edx:eax` register pair.
- For 64-bit operands, the value of the `rax` register is multiplied, and the result is stored in the `rdx:rax` register pair.
- `div` and `idiv` - Unsigned and signed division. Both instructions take a single operand.
- For 8-bit operands, the value of the `ax` register is divided by the operand, the quotient is stored in the `al` register, and the remainder is stored in the `ah` register.
- For 16-bit operands, the register pair `dx:ax` is divided, the quotient is stored in the `ax` register, and the remainder is stored in the `dx` register.
- For 32-bit operands, the register pair `edx:eax` is divided, the quotient is stored in the `eax` register, and the remainder is stored in the `edx` register.
- For 64-bit operands, the register pair `rdx:rax` is divided, the quotient is stored in the `rax` register, and the remainder is stored in the `rdx` register.
- `inc` - Increments the value of the first operand.
- `dec` - Decrements the value of the first operand.
- `neg` - Negates the value of the first operand.
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