x86 Instruction Set

revise and note what's before!

Multiplication and division

Multiplication

• Single argument -> result 2x longer than source:
  MUL - unsigned
  IMUL - signed
• Two arguments -> register, register/memory
  result of the same length as source arguments
• Three arguments -> register, register/memory, immediate
  result of the same length as source arguments
  The lower half of result is the same for unsigned and signed multiplication
  The 2- and 3-argument instruction is IMUL but may be used for both unsigned and signed
  MULX - new instruction, 3 arguments, doesn't set flags

Division

only single argument form with implied arguments
complementary to single argument multiplication
dividend 2x longer than divisor and result
two variants:

• DIV - unsigned
• IDIV - signed

Single argument multiplication

• *Division of numbers - ex. MOV EDX 0
• IDIV - sign extension using special no-argument instructios: CBW, CWD, CDQ
  Division instructions generate trap if overflow occurs
• DIV, IDIB and AAM 0 instructions
• usually when we forget to expand the dividend

Operations on flag register

• CLC, STC, CMC
• LAHF, SAHF

Jump instructions

User to transfer the constrol from one contigous addres

Jump

Constant (immediate) target address - unconditional and conditional

Sobroutine returns
RET - return
RET n - return and deallocate arguments

Variable target address - JMP & CALL ex.

• MP EAKX
• JMP [code_pointer]
• JMP [Table + EAX*4]

Conditional instructions other than jumps

CMOVcc - conditional move
SETcc - convert condition to Boolean value 0/1

Instructions for HLL support

RET n - subroutine return with argument deallocation
...

x86 iterative instructions

5 instructions (+2 privileged) using implied registers

• RAX/EAX/AX/AL - data register (a)
• RDX/ECX - loop counter
• RSI/ESI - source pointer (s)
• RDI/EDI - destination pointer (d) - uses ES selector, cannot be altered2
  2 forms:
• single - performs operation, modifies pointer(s)
• iterated - instruction forms while() loop iterated EXC time (while equal/not equal)
• iterated form prefixed with
  REP (repeat) - data move instructions
  REPE (repeat while equal)
  REPNE (repeat while not equal)

Pointers are modified after data access, depending on setting of DF flag in EFLAGS

• DF = 0 - incremented, DF = 1 - decremented
• DF value may be set using CLD and STD instructions
• calling conventions require DF = 0 during every control transfer (CALL, RET)
• Instruction name must be appended with data size specifier (B, W, D, Q)

Instructions
LOAD - load - $a=s\pm \pm$
STOS - store - $\ast d\pm \pm =a$
MOVS - copy - $\ast d\pm \pm =\ast s\pm \pm$
SCAS - scan - $\ast d\pm \pm -a$
CMPS - compare - $\ast d\pm \pm -\ast S\pm \pm$

Self looping instructions

...

CPUID instruction

Before execution, EAX is loaded with leaf identifies (32-bit arg. for CPUID)
After execution, EAX, EBX, ECX, EDX contain useful information (changes depending on EAX leaf value)

• Processor name and version
• Units and instruction available (FPU, MMX, SSE...SSE5, ADM64)
• System programming model extensions
• Cache and TLB capacity and organization details
• Implemented address size
• Processing speed and power