CMPT 295 - Unit - Machine-Level Programming

Lecture 21:

• Assembly language
• Array
• 2D

Last lecture

• Recursion
  • Handled without special instruction
    • Stack frames
    • x86-64 Function call and Register saving conventions
• Manipulation of arrays – in x86-64
  • From x86-64’s perspective, an array is a contiguously allocated region of n * L bytes in memory where L = sizeof( T ) and T -> data type of elements stored in array
• Compute memory address of each array element
  • $$A[i] = A + i * L$$

Today’s Menu

• Introduction
  • C program -> assembly code -> machine level code
• Assembly language basics: data, move operation
  • Memory addressing modes
• Operation leaq and Arithmetic & logical operations
• Conditional Statement – Condition Code + cmovX
• Loops
• Function call – Stack
  • Overview of Function Call
  • Memory Layout and Stack - x86-64 instructions and registers
  • Passing control
  • Passing data – Calling Conventions
  • Managing local data
  • Recursion
• (highlighted) Array (cont’d)
• Buffer Overflow
• Floating-point operations

2D Array

Visual representation:

• A[0][0] … A[0][C-1]
• A[1][0] … A[1][C-1]
• …
• A[R-1][0] … A[R-1][C-1]

T A[P][C]:

• (in C) is a 2D array of data type T, R rows, C columns
• (in x86-64) is a contiguously allocated region of $R \times C \times L$ bytes in memory where L = sizeof(T)

Array layout in memory: Row-major ordering -> Example using int A[R][C]:

• A[0][0]
• …
• A[0][C-1]
• A[1][0]
• …
• A[1][C-1]
• …
• A[R-1][0]
• …
• A[R-1][C-1]

Accessing a row of 2D array

T A[R][C];

• A[i] is an array of C elements (row i of array A)
• Memory address of each row A[i]: A + (i * C * L)
  • where A is base memory address
• Can access other rows by incrementing A by i * C * L
• Example using int A[R][C];

Cells	Row	Memory Arithmatic
A[0][0]	A[0]	A
…	A[0]
A[0][C-1]	A[0]
A[1][0]	A[1]	A + (1*C*4)
…	A[1]
A[1][C-1]	A[1]
…
A[R-1][0]	A[R-1]	A + ((R-1)*C*4)
…	A[R-1]
A[R-1][C-1]	A[R-1]

Accessing an element of 2D array

T A[R][C];

*A[i][j] is element of type T, which requires L bytes *Memory address of each element A[i][j]: A + (i * C * L) + (j * L) = A + (i * C

• j) * L

*Example using int A[R][C]; A[0] A[0][0]

A 6

…

A[R-1]

A[i] A[0][C-1] …

…

A+(iC4)

A[i][j]

…

… A[R-1][0]

…

A+((R-1)C4)

A[R-1][C-1]

Homework

Example: int A[3][5];

• In memory: 0

1

2

3

4

5

6

7

8

9

10

11

12

• Let’s compute the memory address to access:

• A[2]:

• A[2][3]:

7

13

14

Demo - Accessing an element of 2D array

8

• copy.s on our course web site

Summary

• Manipulation of 2D arrays – in x86-64

*From x86-64’s perspective, a 2D array is a contiguously allocated region of R * C * L bytes in memory where L = sizeof( T ) and T -> data type of elements stored in array *2D Array layout in memory: Row-Major ordering *Memory address of each row A[i]: A + (i * C * L) *Memory address of each element A[i][j]: A + (i * C * L) + (j * L)

=> A + (i * C 9

• j) * L

Next Lecture

• Introduction

• C program -> assembly code -> machine level code

• Assembly language basics: data, move operation

• Memory addressing modes

• Operation leaq and Arithmetic & logical operations

• Conditional Statement – Condition Code + cmovX
• Loops
• Function call – Stack

• Overview of Function Call

• Memory Layout and Stack - x86-64 instructions and registers
• Passing control
• Passing data – Calling Conventions

• Managing local data

• Recursion

• Array
• Buffer Overflow

10

• Floating-point operations