Chapter 3B PIC18AssemblyLanguageProgrammingPartB

8/19/2019 Chapter 3B PIC18AssemblyLanguageProgrammingPartB

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3.4 Subroutine and Stack

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Subroutine

• A subroutine is a sequence of

instructions that can be called from

different places in a program.

• Two reasons for creating subroutines: – The problem is too big: Easier to divide the

problem into smaller subproblems

–

There are several places in a program thatneed to perform the same operation.

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!alling subroutine: "s it #ust branching$

• !alling subroutine involves the %#umping& part' which is the same as branching.

• (ut we need to get back to the main program after the subroutine returns.

• Thus' we need a location in which the return address can be stored.

3Image courtesy of S. Katzen, The essential PIC18 Microcontroller, Sringer


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)eturn Address Stack

• The program counter is pushed to the top ofthe return address stack when calling a

subroutine.

•

3*wordb+,*bit memor+• -bit stack pointer STKPTR / pointing to the

top of stack TOS /

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)eturn Address Stack 0ointer ST10T)/

• ST10T) is 2 when powered up• ST10T) is * upon the first subroutine

call.

• Thus' the stack has 3* spaces foraddress storage. – seful for nested subroutine calls.

• The TS address is readable andwritable through three registers: TS'

TS5' TS6.

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7emonstration: Subroutine7emo.asm

• 8ou can run this demonstration +ourself at homeb+ following these instructions: – 7ownload the Subroutine7emo.asm file available at

(lackboard.

–

!reate a new pro#ect in 906A(. or instructions onhow to do this' refer to Tutorial ;eek 3/.

– Step through the program and stop when the green

arrow is pointing to the call instruction.

– pen the 5ardware Stack window b+ selecting


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call instruction

• Encodes subroutineaddress b+ absolute

address similar to

goto/.

• "ncrement stack

pointer ST10T)/

• Top of )eturn

Address Stack TS/= address of ne>t

instruction following

the call instruction.$


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return instruction

• 0ut Top of )eturnAddress Stack TS/

to 0rogram !ounter

0!/.

• 7ecrement stack

pointer ST10T)/

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?ested subroutines

;hat happens if " call another subroutineS),/ within a subroutine S)*/$

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Image courtesy of S. Katzen, The essential PIC18 Microcontroller,

Sringer


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A demonstration on nested subroutines

• ;rite a program to calculate the value off2>2@/ where – f>/ = g>2>*B/ 2>2-

– g>/ = >2>*,

• )eferring to the Subroutine7emo.asm

file uploaded to (lackboard' determine

the status of the stack and the values ofTS' and ST10T) at 0oints *4.

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8ou should be able to ....

• !ode 0"! subroutine.

• 7escribe the stack and its use in

subroutine.

• 7iscuss how branching and subroutine

calling affect the pipelining mechanism.

• 7escribe the difference of the call and

rcall instructions.

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Chapter 3B PIC18AssemblyLanguageProgrammingPartB