Computer Organization and Assembly Language Programming

1. Computer Organizationand Assembly Language Programming CEG 320/520: Computer Organization and Assembly Language Programming

2. High level code Compiler CPU Memory I/O Assembly code Assembler Machine code CPU Assembly Language Programming Computer Organization & • CPU • ALU • Registers • Control logic • Memory • Organization • Cache • Virtual Memory • I/O • Organization of an I/O device • Polled, Interrupt driven, DMA We will write assembly language code, and assemble it into machine code. We will examine in detail how the CPU executes that code. CEG 320/520: Computer Organization and Assembly Language Programming

3. Computer Organization Very Brief Introduction CEG 320/520: Computer Organization and Assembly Language Programming

4. Intro to Computer Organization: Goals • General • Basic types of computer operations • Instruction cycle • Fetch-execute-store cycle • Data Representation • Binary, hex, 2’s complement • Memory organization • Big Endian vs Little Endian • BYTE, WORD, LONG WORD • How memory is addressed in the 68000 • Registers • Defining feature of a Von Neumann machine • Register size and how bits are numbered in the 68000 • Reading: • General Computer Organization: HVZ Chapter 1, 7.1, 7.2 • Memory: HVZ Chapter 2.2 CEG 320/520: Computer Organization and Assembly Language Programming

5. Intro to Computer Organization: Von Neumann • The Von Neumann model of computer processing • Stored Program: Program and data are both stored as sequences of bits in the computer’s memory. • The program is executed one instruction at a time under the direction of the control unit. • The instruction-execution cycle • Instruction Fetch (IF) stage • Get next instruction from the memory address in the program counter (PC) • Place the new instruction in the Instruction Register (IR) • Increment the PC to the next instruction address • Execute operation (EX) stage • Execute the operation specified by the machine instruction in the IR • Branch instructions may update the PC • Repeat CEG 320/520: Computer Organization and Assembly Language Programming

6. Intro to Computer Organization: Instructions • Machine-level instructions either move data and/or addresses, or do simple math or logic operations on data and/or addresses. • Data and/or addresses can be moved to or from memory, and in and out of CPU registers. • Addresses are really just a special type of data. • There are three basic types of computer instructions • Arithmetic Instructions: operate on values stored in memory or registers • Arithmetic, Shift, and Logic instructions • Move Instructions: move data between memory and registers • Load/Store instructions • Move/Copy portions of memory • Branch Instructions: select one of two possible next instructions to execute • Branch on condition, Unconditional branch (Jump) CEG 320/520: Computer Organization and Assembly Language Programming

7. Intro to Computer Organization: Data Representation • Binary • Any unsigned whole number can be represented in base 2 (binary) as a string of 1’s and 0’s. • A binary number can be thought of as a series of “bits” (binary digits), each of which can have the value 1 or 0: • 4-bit example: 01012 = 510 • Range of representation • N-bit binary numbers represent the range of numbers from 0 … 2n-1 • 4-bit binary numbers represent 0 … +16. • There are 10 kinds of people in the world: those who understand binary and those who don’t. CEG 320/520: Computer Organization and Assembly Language Programming

8. Intro to Computer Organization: Data Representation • Hexadecimal • Easier and more compact for humans to view, write, and understand. • There is a one-to one mapping from binary to hexadecimal (hex), where each 4 bits represents one hexadecimal digit. • Example: 0101001110112 = 53B16 • For this reason, computer memory and register contents are usually shown in hex. • Note: base is generally indicated only when it is not obvious from context. CEG 320/520: Computer Organization and Assembly Language Programming

9. Intro to Computer Organization: Data Representation • 2’s Complement - Signed number representation • Positive numbers: • Sign (leftmost) bit = 0. • Magnitude: binary representation of number. • Negative numbers: • Sign (leftmost) bit = 1. • Magnitude: complement binary representation to form 1’s complement, then add 1. • Range of Representation: • For n-bit 2’s complement, range of numbers that may be represented is -2n-1 … 2n-1 – 1. • 4-bit 2’s complement represents numbers -8 … +7. CEG 320/520: Computer Organization and Assembly Language Programming

10. Intro to Computer Organization: Data Representation • 2’s Complement - Why? • Only one representation for ‘0’ • Signed magnitude: 00 (positive 0) or 10 (negative 0) • 1’s complement: 00 (positive 0) or 10 (negative 0) • 2’s complement: 00 • Arithmetic is simplified • Addition is performed identically for positive and negative numbers • Carry generated by adding sign bits can be thrown away CEG 320/520: Computer Organization and Assembly Language Programming

11. Intro to Computer Organization: Data Representation • 2’s Complement – Hexadecimal • Converting a 2’s complement number to a hexadecimal representation merely involves breaking up the 2’s complement representation into 4-bit binary chunks, and converting those to hexadecimal. • Don’t get fancy! Don’t convert the 2’s complement to unsigned binary then convert that to hexadecimal! You will get the wrong value. 2’s complement: 1100 0010 0100 1010 1001 Hexadecimal: C 2 8 A 9 CEG 320/520: Computer Organization and Assembly Language Programming

12. Intro to Computer Organization: Memory • Most basic operation: getting information from memory to the CPU and back. • Conceptually, computer memory is simply a collection of locations where information can be stored as bits. • Most often, memory is byte-addressable. This means it is divided into bytes (8-bit quantities) each identified by a unique address. • Generally, bytes are addressed sequentially, beginning with address 0. CEG 320/520: Computer Organization and Assembly Language Programming

13. Intro to Computer Organization: Memory • Each byte has its own address. CEG 320/520: Computer Organization and Assembly Language Programming

14. Intro to Computer Organization: Memory • A WORD is the basic unit of information • Most CPU operations take place using words • The word-length is the amount of information retrieved in a single memory access. • It is usually easiest to think of memory as a collection of words • Pentium – 32-bit words • Motorola 68000 – 16-bit words • 68030 – 32-bit words • Dec Alpha – 64-bit words CEG 320/520: Computer Organization and Assembly Language Programming

15. Intro to Computer Organization: Memory • Note that 16-bit word addresses are all divisible by 2. • The word at address n contains the bytes at addresses n and n+1. • Big Endian addressing: left byte is considered at address n. (makes sense to look at) • Little Endian: the opposite is true. (easy to get the low-order byte of a word) CEG 320/520: Computer Organization and Assembly Language Programming

16. Intro to Computer Organization: Memory • The Motorola 68000 memory addressing: • Memory is divided into 16-bit words, each having a unique even address (0,2,4…). • Bytes are addressed using the Big Endian scheme. • A long word is a 32-bit value which can begin at any word address. CEG 320/520: Computer Organization and Assembly Language Programming

17. Intro to Computer Organization: Registers • Registers are temporary storage locations • Much faster than memory (10–100×) • Inside the CPU, gated directly to components • Special registers: program counter (PC), instruction register (IR), memory address register (MAR), memory data register (MDR) • Data is typically transferred to a register before operating on it, then results are stored back to memory. • Motorola 68000 • Eight data registers named D0-D7 • Eight address registers named A0-A7 • A7 is a special register - the stack pointer (SP) • Program Counter (PC) • Status Register (SR) CEG 320/520: Computer Organization and Assembly Language Programming

18. Intro to Computer Organization: Registers • Data registers are 32 bits wide. • Operations on data registers can be performed on BYTE, WORD, or LONG word values. • Registers are accessed like Little Endian memory • WORD operations use only bits 15-0. • BYTE operations use only bits 7-0. • Address registers are 32 bits wide. • Only WORD and LONG word operations can be performed on address registers. • The 68000 uses just 24-bits to address memory, but we will always consider addresses to be LONG words. 31 30 29 28 27 26 25 … 15 14 13 12 11 10 … 7 6 5 4 3 2 1 0 CEG 320/520: Computer Organization and Assembly Language Programming

19. Intro to Computer Organization: You Should Know • General • Basic types of computer operations • Instruction cycle • Fetch-execute-store cycle • Data Representation • Binary, hex, 2’s complement • Memory organization • Big Endian vs Little Endian • BYTE, WORD, LONG WORD • How memory is addressed in the 68000 • Registers • Defining feature of a Von Neumann machine • Register size and how bits are numbered in the 68000 • Reading: • General Computer Organization: HVZ Chapter 1, 7.1, 7.2 • Memory: HVZ Chapter 2.2 CEG 320/520: Computer Organization and Assembly Language Programming