Assembly Language Programming By Ytha Yu, Charles Marut Chap 6 (Flow Control Instructions,)

Microprocessor Based Systems
Spring 2013
Department of Electrical Engineering
University of Gujrat

IBM
CHARACTER
DISPLAY
TITLE IBM CHARACTER DISPLAY
.MODEL SMALL
.STACK 100H
.CODE
MAIN PROC
MOVAH, 2 ; display char function
MOVCX, 256 ; no. of chars to display
MOVDL, 0 ; DL has ASCII code of null char
PRINT_LOOP:
INT 21h ; display a char
INC DL ; increment ASCII code
DEC CX ; decrement counter
JNZ PRINT_LOOP ; keep going if CX not 0
; DOS exit
MOVAH, 4CH
INT 21H
MAIN ENDP
END MAIN
3
Label

IBM Character Display
• IBM.ASM
6

Conditional Jumps
• Jxxx destination_label
• In IBM.ASM, the CPU executes JNZ
PRINT_LOOP by inspecting ZF.
• If ZF = 0, control transfers to PRINT_LOOP
• If ZF = 1, it goes on to execute MOV AH, 4CH
• Jump instructions themselves do not affect
the flags.
7

Range of a conditional jump
• destination_label must precede the jump
instruction by no more than 126 bytes, or
follow it by no more than 127 bytes.
8

jumps
Conditional
jumps
Signed Jumps
Unsigned
Jumps
Single Flag
Jumps
Unconditional
jumps
9

The CMP (compare) Instruction
• CMP destination, source
• CMP is just like SUB, except that destination is
not changed.
• CMP AX, BX ; AX = 7FFFh, BX = 0001h
JG BELOW ; AX – BX = 7FFEh
• The jump condition for JG is satisfied because
ZF = SF = OF = 0, so control transfers to label
BELOW.
10

Interpreting the Conditional Jumps
• CMP AX, BX
JG BELOW
• If AX is greater than BX (in a signed sense), then
JG (jump if greater than) transfers to BELOW.
• DEC AX
JL THERE
• If the contents of AX, in a signed sense, is less
than 0, control transfers to THERE.
11

12
Jumps Based on Specific Flags

13
Jumps Based on Unsigned
Comparisons

14
Jumps Based on Signed Comparisons

Signed Versus Unsigned Jumps
• CMP AX, BX ; AX = 7FFFh, BX = 8000h
JA BELOW
• 7FFFh > 8000h in a signed sense, the program
does not jump to BELOW.
• 7FFFh < 8000h in an unsigned sense, and we
are using the unsigned jump JA.
15

Suppose AX and BX contain signed numbers.
Write some code to put the biggest one in CX.
MOV CX, AX ; put AX in CX
CMP BX, CX ; is BX bigger?
JLE NEXT ; no, go on
MOV CX, BX ; yes, put BX in CX
NEXT:
16

The JMP Instruction
• JMP destination
• JMP can be used to get around the range
restriction of a conditional jump.
17

Unconditional Jump
TOP:
; body of the loop
JNZ TOP ; keep looping if CX > 0
MOV AX, BX
; the loop body contains so many instructions
that label TOP is out of range for JNZ
(more than 126 bytes before JMP TOP)
18

Unconditional Jump
TOP:
; body of the loop
JNZ BOTTOM ; keep looping if CX > 0
JMP EXIT
BOTTOM:
JMP TOP
EXIT:
MOV AX, BX
19

High level language constructs
20

IF-THEN
IF condition is true
THEN
execute true-branch statements
END_IF
21

Replace the number in AX by its
absolute value.
IF AX < 0
THEN
replace AX by –AX
END_IF
22

Replace the number in AX by its
absolute value.
; if AX < 0
CMP AX, 0 ; AX < 0 ?
JNL END_IF ; no, exit
; then
NEG AX ; yes, change sign
END IF:
23

IF-THEN-ELSE
IF condition is true
THEN
execute true-branch statements
ELSE
execute false-branch statements
END_IF
24

Suppose AL and BL contain extended ASCII
characters. Display the one that comes first
in the character sequence.
IF AL <= BL
THEN
display the character in AL
ELSE
display the character in BL
END_IF
25

Suppose AL and BL contain extended ASCII
characters. Display the one that comes first
in the character sequence.
MOV AH, 2 ; prepare to display
; if AL <= BL
CMP AL, BL ; AL <= BL?
JNBE ELSE_ ; no, display char in BL
; then ; AL <= BL
MOV DL, AL ; move char to be displayed
JMP DISPLAY ; go to display
ELSE_: ; BL < AL
MOV DL, BL
DISPLAY:
INT 21h ; display it
END_IF
26
ELSE is a
reserved word
Needed to skip false
branch (not needed in
high level language)

CASE
CASE expression
value 1 : statements_1
value 2 : statements_2
.
.
.
value n : statements_n
END_CASE
27

If AX contains a negative number, put –1 in
BX; if AX contains 0, put 0 in BX, if AX
contains a positive number , put 1 in BX.
CASE AX
<0 : put –1 in BX
=0 : put 0 in BX
>0 : put 1 in BX
END_CASE
28

If AX contains a negative number, put –1 in
BX; if AX contains 0, put 0 in BX, if AX
contains a positive number , put 1 in BX.
; case AX
CMP AX, 0 ; test AX
JL NEGATIVE ; AX < 0
JE ZERO ; AX = 0
JG POSITIVE ; AX > 0
NEGATIVE:
MOV BX, -1 ; put -1 in BX
JMP END_CASE ; and exit
ZERO:
MOV BX, 0 ; put -0in BX
JMP END_CASE ; and exit
POSITIVE:
MOV BX, 1 ; put 1 in BX
END_CASE:
29
Only one cmp is
needed as jump
instructions don’t
affect the flags

If AL contains 1 or 3, display “o”;
If AL contains 2 or 4, display “e”.
CASE AL
1, 3 : display “o”
2, 4 : display “e”
END_CASE
30

; case AL
; 1,3 :
CMP AL, 1 ; AL = 1?
JE ODD ; yes, display ‘o’
CMP AL ,3 ; AL = 3?
JE ODD ; yes, display ‘o’
; 2,4 :
CMP AL, 2 ; AL = 2?
JE EVEN ; yes, display ‘e’
CMP AL, 4 ; AL = 4?
JE EVEN ; yes, display ‘e’
JMP END_CASE ; not 1..4
31

ODD: ; display ‘o’
MOV DL, ‘o’ ; get ‘o’
JMP DISPLAY ; go to display
EVEN: ; display ‘e’
MOV DL, ‘e’ ; get ‘e’
DISPLAY:
MOV AH, 2
INT 21H ; display char
END_CASE:
32

Branches with Compound Conditions
• Some times the branching in an IF or CASE
takes from;
• condition_1 AND condition_2
or
condition_1 OR condition_2
33

AND Conditions
• condition_1 AND condition_2
• An AND condition is true if and only if
condition_1 and condition_2 are both true.
• If either condition is false, then the whole
thing is false.
34

Read a character, and if it’s an
uppercase letter, display it.
Read a character (into AL)
IF (‘A’ <= character) and (character <= ‘Z’)
THEN
display character
END_IF
35

Read a character, and if it’s an
uppercase letter, display it.
; read a character
MOV AH, 1 ; prepare to read
INT 21H ; char in AL
; if (‘A’ <= char) and (char >= ‘Z’)
CMP AL, ‘A’ ; char >= ‘A’?
JNGE END_IF ; no, exit
CMP AL, ‘Z’ ; char <= ‘Z’?
JNLE END_IF ; no, exit
; then display char
MOV DL, AL ; get char
INT 21H ; display char
END_IF:
36

OR Conditions
• condition_1 OR condition_2
• condition_1 OR condition_2 is true if at least
one of the conditions is true.
• It is only false when both conditions are false.
37

Read a character, and if it is “y” or “Y”, display
it; otherwise, terminate the program.
Read a character (into AL)
IF (character = ‘y’) or (character = ‘Y’)
THEN
display it
ELSE
terminate the program
END_IF
38

; read a character
; if (character = ‘y’) or (character = ‘Y’)
CMP AL, ‘y’ ; char = ‘y’?
JE THEN ; yes, go to display it
CMP AL, ‘Y’ ; char = ‘Y’?
JE THEN ; yes, go to display it
JMP ELSE_ ; no, terminate
39

THEN:
MOV DL, AL ; get char
INT 21H ; display it
JMP END_IF ; end exit
ELSE_:
MOV AH, 4CH
INT 21H ; DOS exit
END_IF:
40

Looping Structures
• A loop is a sequence of instructions that is
repeated .
• The number of times to repeat may be known
in advance
or
• Depend on some condition
41

FOR LOOP
Loop statements are repeated a known number
of times;
FOR loop_count times DO
statements
END_FOR
42

The LOOP instruction
• LOOP destination_label
; initialize CX to loop_count
TOP:
; body of the loop
LOOP TOP
43

The LOOP instruction
• The counter for the loop is the register CX which
is initialized to loop_count.
• Execution of the LOOP instruction causes CX to be
decremented automatically.
• If CX is not 0, control transfers to
destination_label.
• If CX = 0, the next instruction after LOOP is done.
• destination_label must precede the LOOP
instruction by no more than 126 bytes.
44

Write a count-controlled loop to
display a row of 80 stars.
FOR 80 times DO
display ‘*’
END_FOR
45

Write a count-controlled loop to
display a row of 80 stars.
MOV CX, 80 ; number of stars to display
MOV AH, 2 ; display character function
MOV DL, ‘*’ ; character to display
TOP:
INT 21h ; display a star
LOOP TOP ; repeat 80 times
46

The instruction JCXZ (jump if CX is zero)
• JCXZ destination_label
JCXZ SKIP
TOP:
; body of the loop
LOOP TOP
SKIP:
47

The instruction JCXZ (jump if CX is zero)
• If CX contains 0 when the loop is entered, the
LOOP instruction causes CX to be
decremented to FFFFh, and the loop is then
executed FFFFh = 65535 more times!
48

WHILE condition DO
statements
END_WHILE
_______________________________________
REPEAT
statements
UNTIL condition
WHILE LOOP and REPEAT LOOP
49
Statements
Condition
Statements
Condition

Write some code to count the number
of characters in an input line.
Initialize count to 0
read a character
WHILE character <> carriage_return DO
count = count + 1
read a character
END_WHILE
50

Write some code to count the number
of characters in an input line.
MOV DX, 0 ; DX counts characters
INT 21H ; character in AL
WHILE_:
CMP AL, 0DH ; CR?
JE END_WHILE ; yes, exit
INC DX ; not CR, increment count
INT 21H ; read a character
JMP WHILE_ ; loop back
END_WHILE_:
51

Write some code to read characters
until a blank is read.
REPEAT
read a character
UNTIL character is a blank
______________________________________________
REPEAT:
; until
CMP AL, ‘ ‘ ; a blank?
JNE REPEAT ; no, keep reading
52

Programming with High-Level Structures
• CAP.ASM
Type a line of text:
THE QUICK BROWN FOX JUMPED.
First capital = B Last capital = X
53
If no capital letter entered,
display
“No capital letter entered”

Read and process a line of text
Read a character
WHILE character is not a carriage return DO
IF character is a capital letter (‘A’ <= character AND character <= ‘Z’)
THEN
IF character precedes first capital
THEN first capital = character
END IF
IF character follows last capital
THEN last capital = character
END IF
END IF
Read a character
END_WHILE
54

Display the results
IF no capitals were typed,
THEN
display “No capitals”
ELSE
display first capital and last capital
END_IF
55

ASCII
Character
Table
FIRST
LAST

CAP.ASM
1(4)
TITLE FIRST AND LAST CAPITALS
.MODEL SMALL
.STACK 100H
.DATA
PROMPT DB 'Type a line of text', 0DH,
0AH, '$'
NOCAP_MSG DB 0DH, 0AH, 'No capitals $'
CAP_MSG DB 0DH, 0AH, 'First capital =
'
FIRST DB '['
DB ' Last capital = '
LAST DB '@ $'
.CODE
MAIN PROC
; initialize DS
MOV AX, @DATA
MOV DS, AX

; display opening message
MOV AH, 9 ; display string function
LEA DX, PROMPT ; get opening message
INT 21H ; display it
; read and process a line of text
MOV AH, 1 ; read char function
WHILE_:
; while character is not a carriage return do
CMP AL, 0DH ; CR?
JE END_WHILE ; yes, exit
; if character is a capital letter
CMP AL, 'A' ; char >= 'A'?
JNGE END_IF ; not a capital letter
CMP AL, 'Z' ; chat <= 'Z'?
JNLE END_IF ; not a capital letter
; then
CAP.ASM
2(4)

CAP.ASM
3(4)
; if character precedes first capital
CMP AL, FIRST ; char < first capital?
JNL CHECK_LAST ; no, >=
; then first capital = character
MOV FIRST, AL ; FIRST = char
; end_if
CHECK_LAST:
; if character follows last capital
CMP AL, LAST ; char > last capital?
JNG END_IF ; no, <=
; then last capital = character
MOV LAST, AL ; LAST = char
; end_if
END_IF:
; read a character
JMP WHILE_ ; repeat loop
END_WHILE:

; display results
MOVAH, 9 ; display string function
; if no capitals were typed
CMPFIRST, '['; first = '['
JNECAPS ; no, display results
; then
LEADX, NOCAP_MSG ; no capitals
JMPDISPLAY
CAPS:
LEADX, CAP_MSG ; capitals
DISPLAY:
INT21H ; display message
; end_if
; dos exit
MOVAH, 4CH
INT21H
MAIN ENDP
END MAIN
CAP.ASM
4(4)

Assembly Language Programming By Ytha Yu, Charles Marut Chap 6 (Flow Control Instructions,)

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Assembly Language Programming By Ytha Yu, Charles Marut Chap 6 (Flow Control Instructions,)