1. Quiz #2 Topics Character codes Intel IA-32 architecture Mostly MASM
Registers, memory addressing
Bytes, words, etc.
Little-endian representation
Floating-point unit
Mostly MASM
General form of a MASM program
Directives (TITLE, INCLUDE, etc.)
Segments (.code, .data, etc.)
Declare variables, constants
Comments
Instruction format
Instructions (mov, add, call, etc.)
Trace MASM code
Convert simple statements to MASM
Assembling, linking, loading, etc.

2. Today’s topics More MASM programming Addressing modes
Conditional control structures
Decision
Repetition

3. IA-32 Operand Modes
Immediate constant or literal, OFFSET (memory address)
Examples: PI equ
size DWORD 10
myName BYTE ”Barney”
mov eax, 10
mov edx, OFFSET myName
Register register contents
Examples: mov eax, 10
add eax, ebx
mov size, eax
Direct memory contents
Examples: mov eax, size
Others (later)
Register indirect, Indexed, Base-indexed, Stack

4. Instruction Operand Notation

5. mov ecx,256 mov edx,OFFSET string
Syntax
Examples
MOV mem,accum
mov total,eax
MOV accum,mem
mov al,string
Notes:
accum means “eax or some valid part of eax”
imm means “a literal or constant”
Syntax
Examples
MOV mem,imm
mov color,7
Syntax
Examples
MOV reg,imm
mov ecx,256 mov edx,OFFSET string

6. mov dh,bh mov edx,ecx mov ebp,esp
Syntax
Examples
MOV reg,reg
mov dh,bh mov edx,ecx mov ebp,esp
MOV mem,reg
mov count,ecx
MOV reg,mem
mov ebx,pointer
Notes:
mem8 means “BYTE”
mem16 means “WORD”
mem32 means “DWORD”
sreg means CS, DS, ES, FS, GS, or SS
Syntax
Examples
MOV sreg,reg16
mov ds, ax
MOV sreg,mem16
mov es,pos1
MOV reg16,sreg
mov ax,ds
MOV mem16,sreg
mov stack_save,ss

7. Invalid MOV statements
.data
bVal BYTE 100
bVal2 BYTE ?
wVal WORD 2
dVal DWORD 5
.code
mov ds,45
mov esi,wVal
mov eip,dVal
mov 25,bVal
mov bVal2,bVal
immediate move to DS not permitted
size mismatch
EIP cannot be the destination
immediate value cannot be destination
memory-to-memory move not permitted

8. Branching execution
Sometimes it is necessary to interrupt sequential instruction execution
EIP is changed
Examples:
Skip ahead (e.g., skip the else block)
Jump backwards (e.g., repeat a section of code)
Call a procedure
Conditional / Unconditional branching
Label required

9. MASM Labels Same rules as other identifiers
May not be any previously defined identifier
Label definition ends with :
Don’t use : when referencing the label
Specifies the memory address of the associated instruction
… just like a variable name
Good practice to put label: on a separate line.

10. Unconditional branching
Instruction format is jmp label
label: should be inside the same procedure
MASM allows jumps to labels in other procedures, but execution will certainly get lost in space.

11. Conditional branching
Used for:
if structures (decisions, alternation)
loop structures (repetition, iteration)
In general, MASM requires you to build your own control structures
Note: in the following discussion, status bits (flags) are
Set (means status bit is set to 1)
Cleared (means status bit is set to 0)

12. loop instruction Instruction format is loop label
label: should be inside the same procedure, before the loop instruction
Used for counted loops. Implements a “for” loop.
Conditional branch
Decrements ecx, if ecx is not zero, branch to label
Problem if ecx is changed inside the loop body
Problem if ecx starts at 0, or ecx becomes negative
Exercise great care when constructing nested “loop” loops

13. loop Example Find sum of integers from 1 to 10
; initialize accumulator, first number,
; and loop control
mov eax, 0
mov ebx, 1
mov ecx, 10
sumLoop: ; add numbers from 1 to 10
add eax, ebx
inc ebx
loop sumLoop
; Print result
call WriteDec
; ...

14. Conditional branching
We need a way to control branching by checking some other types of conditions
Examples:
Some repetitive tasks can not be counted in advance
IF-THEN-ELSE structures
MASM provides a way to compare two operands

15. CMP Instruction Compares the destination operand to the source operand
Non-destructive subtraction of source from destination (destination operand is not changed)
Syntax: CMP destination, source

16. CMP Instruction (unsigned)
Example: destination is equal to source
mov al,5
cmp al,5 ; Zero flag set

17. CMP Instruction (unsigned)
Example: destination < source
mov al,4
cmp al,5 ; Carry flag set

18. CMP Instruction (unsigned)
Example: destination > source
mov al,6
cmp al,5 ; ZF = 0, CF = 0
(both the Zero and Carry flags are cleared)

19. CMP Instruction (signed)
The comparisons shown here are performed with signed integers.
Example: destination > source
mov al,5
cmp al,-2 ; Sign flag == Overflow flag
Example: destination < source
mov al,-1
cmp al,5 ; Sign flag != Overflow flag

20. Conditional Jumps Jumps Based On . . . Specific flags Equality
Unsigned comparisons
Signed Comparisons

21. Jcond Instruction
A conditional jump instruction branches to a label when specific register or flag conditions are met
Usually the next instruction after cmp
Examples:
JB, JC jump to a label if the Carry flag is set
JE, JZ jump to a label if the Zero flag is set
JS jumps to a label if the Sign flag is set
JNE, JNZ jump to a label if the Zero flag is cleared
JECXZ jumps to a label if ECX equals 0

22. Jumps Based on Specific Flags

23. Jumps Based on Equality

24. Jumps Based on Unsigned Comparisons

25. Jumps Based on Signed Comparisons

26. Compare and Jump
Task: Jump to a label if unsigned EAX is greater than EBX
Solution: Use CMP, followed by JA
cmp eax,ebx
ja Larger
Task: Jump to a label if signed EAX is greater than EBX
Solution: Use CMP, followed by JG
cmp eax,ebx
jg Greater

27. Compare and Jump
Jump to label L1 if unsigned EAX is less than or equal to Val1
cmp eax,Val1
jbe L1 ; below or equal
Jump to label L1 if signed EAX is less than or equal to Val1
cmp eax,Val1
jle L1

28. Conditional Loop Instructions (Use after CMP)
LOOPZ and LOOPE
Syntax:
LOOPE destination
LOOPZ destination
Logic:
ECX  ECX – 1
if ECX > 0 and ZF=1, jump to destination

29. Conditional Loop Instructions (Use after CMP)
LOOPNZ and LOOPNE
Syntax:
LOOPNZ destination
LOOPNE destination
Logic:
ECX  ECX – 1;
if ECX > 0 and ZF=0, jump to destination

30. Conditional Directives
.IF, .ELSE, .ELSEIF, and .ENDIF
.WHILE, .ENDW
.REPEAT, .UNTIL
Not required for this course
It’s OK to use these in programming assignments, but you must know the “hard way” for exams and quizzes

31. Block-Structured IF Statements
You can create assembly language control structures that are equivalent to statements written in C++/Java/etc..
Example:
mov eax,op1
cmp eax,op2
jne L1
mov X,1
jmp L2
L1: mov X,2
L2:
if( op1 == op2 )
X = 1;
else
X = 2;

32. Assembly Language Control Structures
Extend the idea to create your own
if-then
if-then-else
if-then-elseif-else
compound conditions
while loop
do-while loop
for loop
nested structures, switch structures, etc.

33. if-then check condition using CMP
if condition is false, jump to endThen
(Note: test for complement of condition)
code for TRUE block
endThen:

34. if-then-else (Method 1)
check condition using CMP
if condition is false, jump to falseBlock
(Note: test for complement of condition)
code for TRUE block
jump to endFalse
falseBlock:
code for FALSE block
endFalse:

35. if-then-else (Method 2)
check condition using CMP
if condition is true, jump to trueBlock
code for FALSE block
jump to endTrue
trueBlock:
code for TRUE block
endTrue:

36. if-then-elseif-else check condition1 using CMP
if condition1 is true, jump to trueBlock1
check condition2 using CMP
if condition2 is true, jump to trueBlock2
code for FALSE block
jump to endBlock
trueBlock1:
code for TRUE block1
trueBlock2:
code for TRUE block2
endBlock:

37. Compound conditions (AND)
check condition1 using CMP
if condition1 is false, jump to falseBlock
check condition2 using CMP
if condition2 is false, jump to falseBlock
code for TRUE block
jump to endBlock
falseBlock
code for FALSE block
endBlock:

38. Compound conditions (OR)
check condition1 using CMP
if condition1 is true, jump to trueBlock
check condition2 using CMP
if condition2 is true, jump to trueBlock
code for FALSE block
jump to endBlock
trueBlock
code for TRUE block
endBlock:

39. Pretest loop (while) initialize loop control variable(s) top:
check condition using CMP
if condition is false, jump to endWhile
code for LOOP BODY
(including loop control update)
jump to top
endWhile:

40. Example pre-test loop Double x until x>1000
; initialize accumulator
mov eax, x
dblLoop: ; Double x while x <= 1000
cmp eax, 1000
ja endLoop
add eax, eax
jmp dblLoop
endLoop:
mov x, eax
; ...

41. Posttest loop (do-while)
top:
code for LOOP BODY
(including loop control update)
check condition using CMP
if condition is true, jump to top

42. Example post-test loop Double x until x>1000
; initialize accumulator
mov eax, x
dblLoop: ; Double x while x <= 1000
add eax, eax
cmp eax, 1000
jbe dblLoop
mov x, eax
; ...
Note: may want initial test for x>1000

43. Various solutions
Any control structure may be implemented in a variety of ways.
Experiment!

44. Questions? Quiz #2 Thursday Program #2 is assigned
Learn the MASM instructions!
Experiment! Experiment!! Experiment!!!