2

Lectures schedule

When ? Topic Lecture

October 20, 2013 Introduction to C Programming in Unix Environment - I

1

October 27, 2013 Introduction to C Programming in Unix Environment - II

2

November 3, 2013 Introduction to Assembly 3

November 17, 2013November 10, 2013

Functions and System Calls (Assembly) 4

Midterm A ( December 4, 2013)

December 8, 2013 Unix Processes 5

December 15, 2013 Programs Execution 6

December 22, 2013 Introduction to script languages (Python) 7

January 5, 2014 Web programming 8

Midterm B (January 15, 2014)

3

From last lecture - Registers

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Pentium has 10 32-bit and 6 16-bit registers

Registers are grouped into: General registers Control registers Segment registers

General registers Data registers Pointer registers Index registers

4

From last lecture - Conditional Jump

Jump if the specified condition is satisfied

j<cond> label ;identifies the condition

The condition being tested is the result of the last arithmetic or logic operation

read_char:mov DL,0. . . (code for reading a character into AL). . .cmp AL,0DH ;compares the character to CRje CR_received ; if equal, jump to CR_receivedinc CL ;otherwise, increment CL andjmp read_char ; go back to read another char.

CR_received:mov DL, AL

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but, the CMP doesn’t save the result, so what really happens ?!!

5

From last lecture - Loops

mov CL,50repeat1:<loop body>dec CLjnz repeat1. . .. . .

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mov ECX,50repeat1:

<loop body>loop repeat1. . . . . .

6

Today

Functions and the Stack Pentium Implementation of the stack Uses of the stack Calling Functions

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7

Pentium Implementation of the Stack

A stack is a last-in-first-out (LIFO) data structure

The top-of-the-stack (TOS) is indicated by ESP register

The key characteristics: Only words (16-bit) or doublewords (32-bit) are saved on the stack The stack grows toward lower memory address (downward) TOS always points to the last inserted data item TOS points to the lower byte of the last inserted word

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8

Pentium Implementation of the Stack

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9

Basic Instructions

push source pop destination

The operands can be a 16-bit or 32-bit general purpose registers, or a word or a doubleword in memory

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10

Basic Instructions - Examples

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push 21ABHpush 7FBD329AH

pop EBX

11

Stack Operations

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12

Use of the Stack

The stack is used for three main purposes

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Temporary Storage of Data

Transfer of Control

Parameter Passing

13

Temporary Storage of Data

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value1 and value2 are in memory We want to exchange their values mov doesn’t work, why ?

14

Transfer of Control

The Pentium provides call and ret instructions

After the call instruction, the EIP points to the next instruction to be executed

The processor pushes the content of the EIP (of the calling function) onto the stack

call proc-name

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<return address >

ESP = ESP – 4ESP = EIPEIP = EIP + d

High

Low

15

Transfer of Control

The ret instruction is used to transfer control from the called procedure to the calling procedure

ret

Note: integral return value of procedures are stored in EAX

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High

Low

<return address>

EIP = ESPESP = ESP + 4

16

Parameter Passing

It is more complicated than that used in high-level languages

The calling procedure first places all the parameters need by the called procedure in the stack

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For example, consider passing two 16-bit parameters to a SUM procedure

push number1push number2call sum

17

Parameter Passing

So, how do we retrieve the parameters now ?

Since the stack is a sequence of memory location ESP+4 points to number2, and ESP+6 to number1

For instance, to read number2 we can invoke:

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mov EBX, [ESP+4]

Are we done ? What type of problems we

would encounter?

18

Parameter Passing The stack pointer is updated by the push and pop instructions

the relative offset changes

A better alternative is to use the EBP register

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mov EBP, ESPmov AX, [EBP+4]

Done?

push EBPmov EBP, ESPmov AX, [EBP+4]

Since every procedure uses the EBP register, it should be preserved

19

Parameter Passing

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push number1push number2call sum

sum:push EBPmov EBP, ESP<SUM CODE>mov ESP, EBPpop EBPret

20

Example section .DATA

string db “ESPL”,0

section .CODEmov EAX, string ;EAX = string[0] pointer push EAXinc EAXpush EAX ;EAX = string[1] pointercall swap

swap:push EBPmov EBP, ESPpush EBX ;save EBX – procedure uses EBXmov EBX, [EBP+12] ; EBX = first character pointerxchg AL, [EBX] ; swap between operandsmov EBX, [EBP+8] ; EBX = second character pointerxchg AL, [EBX]mov EBX, [EBP+12] ; EBX = first character pointer

xchg AL, [EBX]pop EBXmov ESP, EBPpop EBPret

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21

Local Variables

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func:push EBPmov EBP, ESPsub ESP, 8...

22

Interrupts

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