MicroprocessorSTACK , LOOP AND SH I FT ING

The Shift InstructionThe 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

The Shift FamilyThere 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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Multiplication & Division by ShiftMultiplication by left shiftA left shift by 1 bit doubles the destination value, i.e. multiplies it by 2.

Division by right shiftA 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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Shift Examples

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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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Stack count

PUSHPUSH: 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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PUSH - Example

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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POP - Example

STACK ApplicationTo store and restore registers

To reverse an input

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Looping StructuresA 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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FOR LOOPThe 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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Example of For LoopFor 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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WHILE LOOPThis loop depend on a condition, in pseudocode,

WHILE condition DO

Statements

END_ WHILE

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Example of WHILE LoopWHILE 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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REPEAT LOOPAnother conditional loop is the REPEAT LOOP. In pseudocode,

REPEAT

Statements

UNTIL condition

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Example of REPEAT LoopREPEAT 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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WHILE Vs REPEATThe 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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