ECE291

ECE291ECE291

Lecture 5Lecture 5

Let’s get stackedLet’s get stacked

ECE 291 Lecture 5ECE 291 Lecture 5

Page Page 22 of 25 of 25

Lecture outlineLecture outline

Program stackProgram stack

Pushing and poppingPushing and popping

Program organizationProgram organization

Debugging hintsDebugging hints

AssignmentsAssignments




Key characteristicsKey characteristics

Stores temporary data during program executionStores temporary data during program execution

One point of access – the top of the stackOne point of access – the top of the stack

Last-in-first-out (LIFO) storageLast-in-first-out (LIFO) storage Data is retrieved in the reverse order from which it Data is retrieved in the reverse order from which it

was storedwas stored

Instructions that directly manipulate the stackInstructions that directly manipulate the stack PUSH – places data on top of stackPUSH – places data on top of stack POP – removes data from top of stack POP – removes data from top of stack




Implementation in memoryImplementation in memory

In use

In use

In use

In use

In use

In use

Free

Free

Free

Free SS

SS:SP

Original SP

Direction of increasing memory addresses

Stack grows in direction of decreasing memory addresses




Implementation in memoryImplementation in memory

SS – Stack segmentSS – Stack segment

SP – Stack pointer always points to the top of the stackSP – Stack pointer always points to the top of the stack SP initially points to top of stack (high memory address)SP initially points to top of stack (high memory address) SP decreases as data is PUSHedSP decreases as data is PUSHed

PUSH AX PUSH AX SUB SP, 2; MOV[SS:SP], AX SUB SP, 2; MOV[SS:SP], AX SP increases as data is POPedSP increases as data is POPed

POP AX POP AX MOV AX, [SS:SP]; ADD SP, 2 MOV AX, [SS:SP]; ADD SP, 2

BP – Base pointer can point to any element on the stackBP – Base pointer can point to any element on the stack



Push examplePush example

6AB3

0800

0300

6A

B3

037FF

03800

037FE

03000

To address 12FFF

Stack segment

SS

SP

DX

CX

BX

AX

Register array

PUSH BX

SP b

efor

e pu

sh

SP afte

r pus

h



Pop examplePop example

392F

1006

0000

39

2F

01007

01008

01006

00000

To address 0FFFF

Stack segment

SS

SP

DX

CX

BX

AX

Register array

POP BX

SP a

fter p

op

SP bef

ore

pop



Push and popPush and pop

PUSH and POP always store or retrieve words PUSH and POP always store or retrieve words of data (never bytes)of data (never bytes)

80386 and above allow words or double words 80386 and above allow words or double words to be transferred to and from the stackto be transferred to and from the stack

Source of data for PUSHSource of data for PUSH Any internal 16-bit register, immediate data, any Any internal 16-bit register, immediate data, any

segment register, or any segment register, or any two bytes of memorytwo bytes of memory

Pop places data intoPop places data into Internal register, segment register (except CS), or a Internal register, segment register (except CS), or a

memory locationmemory location



Pusha and popaPusha and popa

80286 and later include PUSHA and 80286 and later include PUSHA and POPA to store and retrieve the contents POPA to store and retrieve the contents internal register set (AX, BX, CX, DX, SP, internal register set (AX, BX, CX, DX, SP, BP, SI, DI)BP, SI, DI)



Stack initialization exampleStack initialization example

Assume that the stack segment resides in Assume that the stack segment resides in memory locations 10000h – 1FFFFhmemory locations 10000h – 1FFFFh

The stack segment is loaded with 1000hThe stack segment is loaded with 1000h

The SP is loaded with 0000hThe SP is loaded with 0000h This makes the stack 64KBThis makes the stack 64KB First PUSH does 0000h – 0002h = FFFEh, First PUSH does 0000h – 0002h = FFFEh,

storing data in 1FFFEh and 1FFFFhstoring data in 1FFFEh and 1FFFFh



Stack useStack use

To storeTo store RegistersRegisters Return address information while procedures Return address information while procedures

are executingare executing Local variables that procedures may requireLocal variables that procedures may require Dynamically allocated memoryDynamically allocated memory

To passTo pass Parameters to procedures (function Parameters to procedures (function

arguments)arguments)



Temporary register storageTemporary register storage

PUSH and POP registers to preserve their valuesPUSH and POP registers to preserve their valuesPUSH AXPUSH AX ; Place AX on the stack; Place AX on the stackPUSH BXPUSH BX ; Place BX on the stack; Place BX on the stack

……

< modify contents of AX and BX >< modify contents of AX and BX >

……

POP BXPOP BX ; Restore original value of BX; Restore original value of BXPOP AXPOP AX ; Restore original value of AX; Restore original value of AX



Temporary register storageTemporary register storage

Why would you want to backup and restore Why would you want to backup and restore registers?registers?Because registers are themselves temporary Because registers are themselves temporary storage for instruction operands or memory storage for instruction operands or memory addressesaddressesYou might need to do a task that modifies You might need to do a task that modifies registers, but you might then need the original registers, but you might then need the original contents of those registers latercontents of those registers laterAny data that is an end result more than likely Any data that is an end result more than likely should go into a memory location (a variable)should go into a memory location (a variable)



The stack and proceduresThe stack and procedures

call proc_namecall proc_name Pushes the instruction pointer and sometimes the Pushes the instruction pointer and sometimes the

code segment register onto the stackcode segment register onto the stack Performs an unconditional jump to the label Performs an unconditional jump to the label proc_nameproc_name

retret Pops saved IP and if necessary saved CS from the Pops saved IP and if necessary saved CS from the

stack and back into the IP and CS registersstack and back into the IP and CS registers This causes the instruction following the call This causes the instruction following the call

statement to be executed nextstatement to be executed next

More on all of this procedure stuff tomorrowMore on all of this procedure stuff tomorrow




Create block structure and/or pseudocode on Create block structure and/or pseudocode on paper to get a clear concept of program control paper to get a clear concept of program control flow and data structuresflow and data structuresBreak the total program into logical Break the total program into logical procedures/macrosprocedures/macrosUse jumps, loops, etc. where appropriateUse jumps, loops, etc. where appropriateUse descriptive names for variablesUse descriptive names for variables Noun_type for typesNoun_type for types Nouns for variablesNouns for variables Verbs for procedures/macrosVerbs for procedures/macros




Good program organization helps with debuggingGood program organization helps with debuggingPrograms do not work the first timePrograms do not work the first timeStrategy to find problemsStrategy to find problems

Use TD and set breakpoints to check program progressUse TD and set breakpoints to check program progress Use comments to temporarily remove sections of codeUse comments to temporarily remove sections of code Use “print” statements to announce milestones in the progrmUse “print” statements to announce milestones in the progrm

Test values and test casesTest values and test cases Try forcing registers or variables to test output of a procedureTry forcing registers or variables to test output of a procedure Use “print” statements to display critical dataUse “print” statements to display critical data

Double-check your logicDouble-check your logicTry a different algorithm if all else failsTry a different algorithm if all else fails



NASM directivesNASM directives

IncludesIncludes %include “drive:\path\filename”%include “drive:\path\filename”DefinitionsDefinitions

DB/RESBDB/RESB define/reserve byte (8 bits)define/reserve byte (8 bits) DW/RESWDW/RESW define/reserve word (16 bits)define/reserve word (16 bits) DD/RESDDD/RESD define/reserve doubleword (32 bits)define/reserve doubleword (32 bits) EQUEQU names a constant (has no effect on names a constant (has no effect on

memory)memory)

LabelsLabels ““.” prefixed.” prefixed “local” to previous non-dotted label“local” to previous non-dotted label ““:” suffix:” suffix not required and doesn’t change labelnot required and doesn’t change label Global.localGlobal.local can access dotted local labels anywhere can access dotted local labels anywhere

bybyprepending previous non-dotted label prepending previous non-dotted label




ProceduresProcedures Labeled sections of code you can jump to and Labeled sections of code you can jump to and

return from any point in your programreturn from any point in your program Procedures begin with merely a non-dotted Procedures begin with merely a non-dotted

labellabel Use dotted labels inside procedures to keep Use dotted labels inside procedures to keep

them local to the procedurethem local to the procedure Helps keep labels unique program-wideHelps keep labels unique program-wide




MacrosMacros Procedures require some overhead in memory and Procedures require some overhead in memory and

execution timeexecution time NASM replaces macro call with the macro code itselfNASM replaces macro call with the macro code itself AdvantagesAdvantages

Faster (no call instruction)Faster (no call instruction)

Readability – easier to understand program functionReadability – easier to understand program function DrawbacksDrawbacks

Using the macro multiple times duplicates codeUsing the macro multiple times duplicates code

Tricky to debug sometimes (especially when you have Tricky to debug sometimes (especially when you have nested macros)nested macros)




References to proceduresReferences to procedures EXTERN EXTERN namename – gives you access to procedures and – gives you access to procedures and

variables in other files (such as library files)variables in other files (such as library files) GLOBAL GLOBAL namename – makes your procedures and – makes your procedures and

variables available to other files (as if you were variables available to other files (as if you were creating a library)creating a library)

Segment definitionSegment definition SEGMENT SEGMENT namename Examples:Examples: SEGMENT STACKSEGMENT STACK

SEGMENT CODESEGMENT CODE



Example program structureExample program structure

; ECE291:MPXXX; ECE291:MPXXX

; In this MP you will develop a program which take input; In this MP you will develop a program which take input

; from the keyboard; from the keyboard

;======;====== ConstantsConstants ==================================================================================================

;ASCII values for common characters;ASCII values for common characters

CR CR EQUEQU 13 13 ; EQU’s have no effect on memory; EQU’s have no effect on memory

LF LF EQUEQU 10 10 ; They are preprocessor directives only; They are preprocessor directives only

ESCKEY ESCKEY EQUEQU 27 27 ; LF gets replace with 10 when assembled; LF gets replace with 10 when assembled

;====== ;====== ExternalsExternals ==================================================================================================

; -- LIB291 Routines; -- LIB291 Routines

externextern dspmsg, dspout, kbdin dspmsg, dspout, kbdin

externextern rsave, rrest, binasc rsave, rrest, binasc



Example program structureExample program structure;==== LIBMPXXX Routines (Your code will replace calls to

these ;functions) extern LibKbdHandlerextern LibMouseHandlerextern LibDisplayResultextern MPXXXXIT

;====== Stack ====================================================stkseg segment STACK ; *** STACK SEGMENT *** resb 64*8 ; 64*8 = 512 Bytes of Stackstacktop:

;====== Begin Code/Data ==========================================codeseg segment CODE ; *** CODE SEGMENT ***



Example program structureExample program structure;====== Variables ==============================================inputValid db 0 ; 0: InputBuffer is not ready

; 1: InputBuffer is ready;-1: Esc key pressed

operandsStr db 'Operands: ','$'

OutputBuffer 16 times db 0 ; Contains formatted output

db ‘$’ ; (Should be terminated with '$')

MAXBUFLENGTH EQU 24InputBuffer MAXBUFLENGTH times db 0

; Contains one line of user inputdb ‘$’

graphData %include “graphData.dat” ; data

GLOBAL outputBuffer, inputValid, operandsStr GLOBAL graphData



Example program structureExample program structure

;====== ;====== ProceduresProcedures ======================================================================================

KbdHandlerKbdHandler

<Your code here> <Your code here>

MouseHandlerMouseHandler

<Your code here><Your code here>

DisplayResultDisplayResult

<Your code here><Your code here>

;====== ;====== Program InitializationProgram Initialization =============================== ===============================

..start:..start:

mov ax, csmov ax, cs ; Use common code & data segment; Use common code & data segment

mov ds, axmov ds, ax

mov sp, stacktopmov sp, stacktop ; Initialize top of stack; Initialize top of stack



Example program structureExample program structure;====== ;====== Main ProcedureMain Procedure ======================================== ========================================MAIN:MAIN:

MOV AX, 0B800h ;Use extra segment to access videoMOV AX, 0B800h ;Use extra segment to access video MOV ES, AXMOV ES, AX

<here comes your main procedure><here comes your main procedure>

CALL MPXXXXIT ; Exit to DOSCALL MPXXXXIT ; Exit to DOS

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