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ADX - x86
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Revision as of 11:28, 10 August 2021 by 24.116.116.50 (talk) (→‎Overview: Changed "an register" to "a register"; changed "effecting flags" to "affecting flags.")
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ADX is an x86 instruction set extension that introduced arbitrary-precision arithmetic operations.

Overview

ADX introduced two new instructions:

  • ADCX
  • ADOX

They are similar to the ADC instruction by doing a add with overflow, but

  • ADCX uses the Carry flag as source and destination of overflow and leaves the other flags untouched
  • ADOX uses the Overflow flag as source and destination of overflow and leaves the other flags untouched

This allows two parallel addition of multi-precision numbers because both have their separate carry flag.

Note:

  • To add a constant to a register without affecting flags, you can use the LEA reg, [reg + n] instruction
    xor  rax, rax
 lbl:
    mov  rbx, [r8 + r15]
    adcx rbx, [r9 + r15]
    mov  [r9 + r15], rbx
    mov  rbx, [r10 + r15]
    adox rbx, [r11 + r15]
    mov  [r11 + r15], rbx
    lea  r15, [r15 + 8]
    dec  r14
    jnz  lbl
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