Control Structures

• This is section 7.7
• Labels
  • As we have discussed these are just addresses
  • The assembler does a nice job of keeping these "abstract" for us.
  • Labels are best if treated like variables, but include the function name.
    • done, else, endif are ok if there is only one loop, if, ...
    • See the sort code for example
    • These add greatly to the readability of your code.
  • In nasm a label starting with a . is associated with the previous label.

    TopOfLoop1:
         test eax, eax,
         jz .done
         ...
         jmp TopOfLoop1
    .done
    
    TopOfLoop2:
         test eax, eax,
         jz .done
         ...
         jmp TopOfLoop2
    
    .done 

  • See 2.9, local labels
• Unconditional jumps jmp label
  • Makes the instruction at label the next instruction to be executed.
• Conditional jumps are based on a two step process
  • Do something that sets a status flag.
  • Jump based on the value of that flag.
• The flag register is a special purpose register.
  • It contains information about the last instruction executed.
  • Called the rFlags in the book, eflags in GDB
  • wikipedia explains that this is like other registers in that r is a 32 bit register and e is a 32 bit register.
  • Pure trivia, but you can do a pushf, pushfd, pushfq to move the flags to the stack.
  • And sahf to save the bottom 8 bits to the ah register.
  • Jorgensen on page 12 gives us a table describing the flags.
    • CF : indicates the last instruction had a carry
    • PF : indicates parity of the last byte
    • ZF : the last operation resulted in 0
    • SF: the last operation produced a negative number
    • OF: the last operation had overflow
  • Wikipedia has more.
    • IF: interrupt enabled,
  • Fortunately we do not have to look at these ourselves right now.
  • But the flag register is observable in gdb (eflags)
• The test instruction
  • Does a bitwise and of the operands.
  • Sets the SF, ZF, PF are modified.
• The cmp instruction
  • Takes two operands.
  • Either, but not both may be memory
  • The first may not be an immediate
  • Subtracts the second from the first.
  • Sets OF, SF, ZF, AF, PF, and CF
  • But does not save the result.
• Conditional jump instructions
  • je, jne (jump equal, not equal) works for all data.
  • jl, jle, jg, jge (jump less, less equal, greater, greater equal) will work for signed data
  • jb, jbe, ja, jae (jump below, below equal, above, above equal) will work for unsigend data.
• Intel has a loop instruction.

•     ; rcx is the LCV
      ;set rcx to be positive
  LABEL: 
      ; loop body code
      loop LABEL 

• This is the same as

      ; rcx is the LCV
      ;set rcx to be positive
  LABEL: 
      ; loop body code
      dec rcx
      cmp rcx, 0
      jne LABEL

• The last three lines are the loop code.
• I would probably never use this.
  • It is very restrictive.
  • It does not make the code clearer (IMHO)
  • It is not in most modern assembly languages.