1.
Microprocessor
S TAC K, L OOP AND SHI F T ING

2.
The Shift Instruction
The shift Instructions shift the bits in the destination operand by one or more
position either to the left or right.
 For a shift instruction, the bits shifted out or lost.
For a single shift, the form is
Opcode destination, 1
For a shift of N positions, the form is
Opcode destination, CL

3.
The Shift Family
There are two different sets of shift instructions
 One set for doubling and halving unsigned binary numbers
SHL (Shift Left) – doubles
SHR (Shift Right) - halves
 The other for doubling and halving signed binary numbers
SAL (Arithmetic Shift Left) – doubles
SAR (Arithmetic Shift Right) – halves
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4.
Multiplication & Division by Shift
Multiplication by left shift
 A left shift by 1 bit doubles the destination value, i.e. multiplies it by 2.
Division by right shift
 A right shift by 1 bit halves it and rounds down to the nearest integer, i.e.
divides it by 2.
 But for a sign interpretation, SAR must be used instead of SHR.
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5.
Shift Examples
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6.
STACK
One dimensional data structure
Items are added and removed from one end of the structure i.e. Top of the Stack
Last In First Out Structure
STACK directive used to reserve a block of memory for stack. The syntax is:
STACK 100h
When program is assembled and load in memory:
SS (Stack Segment) Register: holds stack Segment Address
SP (Stack Pointer) Register: initialized to the value specified with the STACK directive, represents
empty stack position
When stack is not empty, SP represents the top of the Stack
Stack grows toward the beginning of the memory.
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7.
Stack count

8.
PUSH
PUSH: Used to add a new source (16-bit register or memory word) on stack
Syntax:
PUSH source
Execution of PUSH causes:
SP is decreased by 2
A copy of source contents is moved to the address specified by SS:SP.
Source remains unchanged
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9.
PUSH - Example

10.
POP
POP: Used to remove top item from stack to destination (i.e. a 16-bit register
or memory word).
Syntax:
POP destination
Execution of POP causes:
The contents of SS:SP (top of the stack) is moved to the destination.
SP is increased by 2
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11.
POP - Example

12.
STACK Application
To store and restore registers
To reverse an input
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13.
Looping Structures
A loop is a sequence of instructions that is repeated.
The Number of times to repeat may be known in advance, or it may depend on
condition.
The counter register CX initialized to loop_count.
Execution of LOOP instruction causes CX to be decremented automatically.
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14.
FOR LOOP
The loop statements is repeated a known number of times.
In pseudo code,
For loop_count times do (using CX)
Statements
END_FOR
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15.
Example of For Loop
For 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
FOR:
INT 21H ; display a star
Loop FOR ; repeat 80 times
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16.
WHILE LOOP
This loop depend on a condition, in pseudocode,
WHILE condition DO
Statements
END_ WHILE
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17.
Example of WHILE Loop
WHILE Loop to count the number of characters in an input line :
MOV DX,0 ; DX counts characters
MOV AH,1 ; prepare to read
INT 21H ; character in AL
WHILE:
CMP AL, 0DH ; Cursor?
JE END_WHILE ; yes, exit
INC DX ; not Cursor, increment count
INT 21H ; read a character
JMP WHILE ; loop back
END_WHILE:
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18.
REPEAT LOOP
Another conditional loop is the REPEAT LOOP. In pseudocode,
REPEAT
Statements
UNTIL condition
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19.
Example of REPEAT Loop
REPEAT Loop to read character until a blank is read :
MOV AH, 1 ; prepare to read
REPEAT:
INT 21H ; char in AL
; until
CMP AL, “ ” ; a blank?
JNE REPEAT ; no, keep reading
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20.
WHILE Vs REPEAT
The advantage of WHILE is that the loop can be bypassed if the terminating
condition is initially false.
Whereas the statements in a REPEAT must be done at least once.
The code for a REPEAT loop is likely to be little shorter as there is only a
conditional jump at the end.
Whereas the WHILE loop is little longer as there has two conditional
jumps at the top and at the end.
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