learning-assembly/ch3/explore_eflags.s at main · abbasovdev/learning-assembly · GitHub

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# PURPOSE: Reset eflags register and broke comparision


# NOTE: How cmpl effects to eflag?
# consider: cmpl edx, ecx
# 1) if ecx < edx, then
#       eflags  0x297   [ CF PF AF SF IF ]
#
# 2) if ecx = edx, then
#       eflags  0x246   [ PF ZF IF ]
#
# 3) if ecx > edx, then
#       eflags  0x202   [ IF ]


# SOURCE: FLAGS REGISTER: https://en.wikipedia.org/wiki/FLAGS_register
#     Abbreviation            Description                 Category            =1                      =0
# 1.  CF                      Carry flag                  Status flag         CY(Carry)               NC(No Carry)
# 2.  PF                      Parity flag                 Status flag         PE(Parity Even)         PO(Parity Odd)
# 3.  AF                      Adjust flag                 Status flag         AC(Auxiliary Carry)     NA(No Auxiliary Carry)
# 4.  ZF                      Zero flag                   Status flag         ZR(Zero)                NZ(Not Zero)
# 5.  SF                      Sign flag                   Status flag         NG(Negative)            PL(Positive)
# 6.  IF                      Interrupt enable flag       Control flag        EI(Enable Interrupt)	  DI(Disable Interrupt)

# CF - when an arithmetic carry or borrow has been generated out of ALU.
# PF - if the numbers of set bits is odd or even in the binary representation of the result of the last operation.
# AF - if carry is generated at 5th position then auxilliary flag is SET to 1.
# ZF - it is set to 1, or true, if an arithmetic result is zero, and reset otherwise.
# SF - it is set to 1, or true, if an arithmetic result is negative, and reset otherwise.
# IF - determines whether or not the CPU will respond to maskable hardware interrupts.


.section .data

.section .text
    .globl _start
        _start:

            movl $5, %ecx                           # set ecx = 5
            movl $88, %edx                          # set edx = 88

            cmpl %edx, %ecx                         # compare edx and ecx (must be ecx < edx). Look at above "NOTE" section.


            # starting reset flags (SF ZF AF PF CF)
            lahf                                    # load flags into ah register
            movb $0, %ah                            # set ah = 0
            sahf                                    # store ah into flags
            # ending reset flags


            jl setzero                              # if ecx < edx, jump setzero
            jmp setone                              # else, jump setone


            setzero:                                # if ecx < edx, then
                movl $0, %ebx                       # set ebx = 0 and
                jmp end                             # jump end of the program


            setone:                                 # if ecx >= edx, then
                movl $1, %ebx                       # set ebx = 1, and
                jmp end                             # jump end of the program


        end:                                        # end of the program
            movl $1, %eax                           # system call number (sys_exit)
            int $0x80                               # call kernel