Flow of Control

1
Flow of Control

• Assembly Language Programming
• University of Akron
• Dr. Tim Margush

2
Unconditional Jump

• JMP operator destination
• Intrasegment direct E9 disp_16
• Intrasegment direct short EB disp_8
• Intrasegment indirect FF mod 100 r/m ??
• Intersegment direct EA disp_16 seg_16
• Intersegment indirect FF mod 101 r/m ??
• operator SHORT, NEAR PTR, or FAR PTR
• NEAR PTR is the usual default

3
Specifying the Jump Target

• Instruction Label
• A symbolic name defined to be an address in the
  code segment of a program
• A label may be attached to any point in the code
  of a program
• a_Label jmp a_Label
• Labels definitions usually have a colon at the
  end signifying them as NEAR

4
Executing a Jump

• Intrasegment jumps are caused by changing the IP
  register to a new value
• Short jumps add a signed 8-bit displacement to IP
• Near jumps add a signed 16-bit displacement to IP
  
• Intersegment jumps change both the CS and IP
  registers
• Far jumps simply assign new values to these
  registers

5
Sample Jump Encodings

• 11060100 EB2A JMP 012C
• 012C-0102002A
• 11060102 EBFC JMP 0100
• 0100-0104FFFC
• 11060104 E97F00 JMP 0186
• 0186-01060080 (too far for short!)
• 0186-0107007F
• 11060107 E9F5FE JMP FFFF
• FFFF-010AFEF5

6
Conditional Jumps

• Jxxx destination
• There are 30 some variations that interrupt
  sequential flow based on various flag settings
• JNZ - Jump if zero flag is clear (0) meaning the
  result of a previous operation was non-zero
• JC - Jump if a previous operation caused the
  carry flag to be set (1)

7
Range of Conditional Jumps

• All conditional jumps are SHORT
• range is -128 to 127 bytes
• 80386 allow larger distances
• Combine a conditional and unconditional jump to
  overcome this range limitation
• jz too_far ugh jnz is_close
• use code at jmp near ptr too_far
• right! is_close

8
Using Conditional Jumps

• Conditional jumps typically follow an instruction
  that alters the flag bits
• CMP destination, source
• Computes (destination-source) and sets flag bits
• result is not stored
• flags allow us to decide lt, lt, gt, gt, , ltgt,
  etc
• we can also interpret the results meaningfully
  for signed or unsigned data

9
Implementing an IF-THEN

• unsigned int n
• if (ngt7) do_it()
• If n is a signed int, use jng (not greater)
• unsigned
• above, below
• signed
• less, greater

• if (ngt7)
• mov ax,n
• cmp ax,7
• jna skip_it
• then-part
• call do_it
• end if
• skip_it

10
Implementing an IF-ELSE

• char n
• if (n'7')
• do_it()
• else
• do_that()
• Document the control structures and keep the
  parts in the usual order

• if (n'7')
• cmp n,'7'
• jne else_
• then-part
• call do_it
• jmp short endif
• else_
• call do_that
• endif

11
Implementing a WHILE

• int n
• while (ngt0) n-2
• This loop could be optimized by keeping n in a
  register and storing to memory only at end of loop

• while (ngt0)
• while_
• cmp n,0
• jle end_while
• loop-body
• sub n,2
• jmp while_
• end_while

12
Compound Conditions

• char n int w,x
• if (ngt'A' wx)
• whatever()
• This example uses short-circuit evaluation
• if the first condition is false it immediately
  skips past the then-part

• if(ngt'A'wx)
• cmp n,'A'
• jl no_go
• mov ax,w
• cmp ax,x
• jne no_go
• then-part
• call whatever
• no_go

13
Compound Conditions - OR

• char n,k unsigned int w
• if (nltgtk wlt10)
• whatever()
• This example uses short-circuit evaluation
• if the first condition is true it immediately
  skips to the then-part

• if(nltgtkwlt10)
• mov ah,n
• cmp ah,k
• jne then_
• cmp w,10
• ja end_if
• then_
• call whatever
• end_if

14
LOOP

• LOOP destination
• decrements CX but does not change any flags
• if CX is not zero after the decrement, control is
  transferred to the destination label
• This is a SHORT jump only

• for (x9xgt0x--) nx
• for(x9xgt0x--)
• mov cx,9
• top_loop
• add n,cx nnx
• loop top_loop

15
JCXZ destination

• Directly compares CX to 0 and jumps to the
  destination if equal
• This instruction does not affect the flags
• It is commonly used to bypass the first iteration
  of a loop if the count is already 0

• while(xgt0)do_it()
• mov cx,x
• jcxz skip_it
• top_loop
• call do_it
• loop top_loop
• skip_it

16
LOOPZ/E and LOOPNZ/E

• Enhancement of the LOOP instruction
• The state of the ZERO Flag may also cause loop
  termination
• Loop while ZF/equal CX!0
• Loop while (NZ/ not equal) CX!0

• Remember that LOOP decrements CX, but this does
  not affect the flags!
• LOOPZ LOOPE
• LOOPNZLOOPNE
• Some action inside the loop should affect the
  zero flag (cmp ?)

17
LOOPZ Example

• This program accepts at most 9 characters from
  the keyboard
• When the 9th character is pressed (or the enter
  key is used) the number of keypresses is displayed

• mov ah,1
• mov cx,9
• next_char
• int 21h
• cmp al,13
• loopne next_char
• determine count
• mov ax, 0239h
• sub al,cl
• mov dl,al
• int 21h

18
Homework

• Page 113
• 1, 3, 4, 10