Week 8

Stack and Subroutines

StackStackThe stack is a section of data memory (or RAM) that is

reserved for storage of temporary dataThe data may represent addresses

The stack operates in a last-in first-out (LIFO) mannerThis means that access to the stack is in a restricted manner,

whereby new data is added to the top of the stack and data is removed from the top also

While it appear that such a structure for storing and retrieving data seems limited, the stack actually makes handling of certain programming tasks very easy

See important uses for stack soon

StackStack

Think of a stack of platesIt is natural to add a plate to the

top of stackIt is natural to remove a plate from

the top alsoThe terminology used for stacks

Push Meaning add to of stack

PopMeaning remove from top of stack

The terminology associated with stacks comes from the spring loaded plate containers common in dining halls.

When a plate is washed it is pushed on the stack.

When someone needs food, a clean plate is popped off the stack.

StackStack

Data is pushed on top and poped from the top of a stack

To access a stack in memory, the address of the top of the stack is needed

Processers have a special register called a Stack Pointer (SP) to store the address of the stack top

Actually processor designers differ in how this is handled.

Some processors have a stack pointer that points to the last data pushed on the stack, while others point to the next available location!

SP

n+5n+4n+3n+2n+1

n

Stack: Example operationStack: Example operation

SP

n+5n+4n+3n+2n+1

n

SP

n+5n+4

4n+3n+2n+1

n

PUSH #4

SP

n+58n+44n+3

n+2n+1

n

PUSH #8

SP

n+58n+44n+3

n+2n+1

n

POP A

8 A

SP

n+515n+44n+3

n+2n+1

n

PUSH 15

8 A

11

SP

n+515n+44n+3

n+2n+1

n

PUSH 11

8 A

Stack: 8051 DetailsStack: 8051 Details8051 has a SP reg

8 bit register in the SFR block (address

Used to address (indirectly) a location in internal data memory

Max of 256 locationsInitialised to address 07H on

power upCan be changed if needed

Stack grows with increasing memory addresses

The two instructions arePUSH direct

Operates by firstly incrementing the contents of the SP and then storing the value addressed by direct at the address in SP

POP directOperates by copying the byte

pointed to by SP into the location specified by direct. The contents of SP is then decremented by 1

8051 Programmer’s Model8051 Programmer’s Model

PC

A

B

R7R6R5R4R3R2R1R0

8-bits

16-bits

SP

128bytes

8 bits 32 bytesused

00h

20h

80h

FFh

128 bytes

80h

FFhSFRs

128bytes

DirectAddressing

InDirectAddressing

Direct &Indirect

Addressing

Note: The SP reg is shown twice here!

ExampleExample

SP

000AH

0006H0007H0008H0009H

A

000AH

000BH000CH000DH000EH

0041H

0038H0039H0040H

Assume SP contains 07H, A contains 3AH and address 40H contains 00H

What does the stack look like after these instructions?

PUSH ACCINC APUSH ACCPOP 39HPUSH 40HADD A,#05HPUSH ACCPOP 40HPOP ACC

ExampleExample

The A, B and PSW registers are required to perform some calculations, but contain values that are needed later. Use the stack to solve the problem

SolutionPUSH ACC ; Store on Stack.PUSH BPUSH PSW…. ; Do calculations…. ; here.POP PSW ; Restore from stack.POP B ; Note order of instructions,POP ACC ; this is important!!

Subroutines

SubroutinesSubroutines

Subroutines provide a mechanisn to reuse a piece of code

They also simplify the design and coding process, allowing the programmer to partition and abstract subtasks

Subroutines also makes testing and debugging easier

Subroutines: ACALL and LCALL

Subroutines: ACALL and LCALL

Just as there was AJMP and LJMP, similarly there is ACALL and LCALL. The assembler will select the approapiate call instruction if the CALL instruction is used (see earlier lectures)

The CALL instruction isCALL addr

Operates by saving the address of the next instruction on the stack and transferring program control to the subroutine code starting at addr

By saving the address of the next instruction, it is possible to resume execution of the next instruction when the subroutine is finished

Subroutines: ExampleSubroutines: Example

MOV P1,10HMOV A,90H

main XRL P1,A CALL Delay

JMP main

Delay:; delay 100ms

MOV R7,#200 ; 200*500us=100msdy1: MOV R6,#250 ; 250*2us=500usdy2: DJNZ R6,dy2 ; here for 500us DJNZ R7,dy1 ; repeat 200 times ; (100ms delay)

RET

When the instruction CALL Delay is executed,

The address of the next instruction is pushed onto the stack automatically

2 address bytes The PC is loaded with the

address of the Delay subroutine

The next instruction to be fetched will then be the first instruction in the Delay subroutine

Subroutines: RETSubroutines: RET

MOV P1,10HMOV A,90H

Main: XRL P1,A CALL Delay

JMP main

Delay:; delay 100ms

MOV R7,#200 ; 200*500us=100msdy1: MOV R6,#250 ; 250*2us=500usdy2: DJNZ R6,dy2 ; here for 500us DJNZ R7,dy1 ; repeat 200 times ; (100ms delay)

RET

How does the processor know when to return from the subroutine?

When a RET instruction is executed

The instruction isRET

It has no operandsThe execution of this

instruction causes the PC to be reloaded with two bytes (16 bits) that are popped off the stack

Example:SimulationExample:

Simulation

SubroutinesSubroutines

Always make sure to include a RET instruction in the subroutine

What are the consequences of not doing so?Because the stack is used, make sure that it is

not corrupted between calling the subroutine and returning

Can still use PUSH and POP instructionsSee next example

SubroutinesSubroutinesWhen subroutines are called, care must be taken not

to corrupt resources that are used by the calling codeCalling code refers to code from which subroutine called

fromCalled code refers to subroutine code itself

A good way to address this issue is to use the stackConsider the Delay subroutine example.

This uses R6 and R7. What if the called function used these registers

Subroutines

Subroutines

MOV P1,10HMOV A,90H

Main: XRL P1,ACALL Delay

JMP main

Delay:PUSH 07PUSH 06; delay 100ms

MOV R7,#200 ; 200*500us=100msdy1: MOV R6,#250 ; 250*2us=500usdy2: DJNZ R6,dy2 ; here for 500us DJNZ R7,dy1 ; repeat 200 times

; (100ms delay)POP 06POP 07RET

By storing the values of R6 and R7 on the stack at start of subroutine and restoring the original values from the stack at the end of the routine, it is now possible to use the delay routine, even when it is called from a section of code that uses R6 or R7 for a different purpose.

This really makes the Delay routine very usable.

Assumes Bank 0 used

SubroutinesSubroutines

Subroutines often require data to be passed in/outThere are 2 standard approaches

Both are flexible in that data does not have to be located at fixed addresses in memory

The two approachesThrough registers

> Limited no. of registers> Fast

Using the stack> More space available> Allows re-entrant operation (recursive calls)

ExamplesExamples

Write a delay routine that can provide a delay of between 100ms and 10s, in increments of 100ms.

Use both approaches for passing data

ExercisesExercises

What is instruction encoding for each of the following?PUSH 41H

POP 41H