Introduction to Assembly Language

1. Introduction to Assembly Language

2. Overview • What is Assembly Language? • Why is AL important to learn? • Machine instructions • AL Instructions • Intel 8086 Architecture • Hands on Activity

3. What is Assembly Language (AL)? • To learn how a computer and its software really work, you need to view them at machine level. • Assembly language is a specific set of instructions for a particular computer system. • AL teaches you about the way the computer’s hardware and OS work together and how application programs communicate with the OS. • It is a programming language with a one-to-one correspondence between its statement and a computer’s main machine language.

4. Cont… • Each computer or family of computers uses a different set of machine instructions and a different assembly language (the computer’s design influences the instructions it can execute). • An assembler – a program that converts or translates source-code programs into machine language, which may in turn be executed by the computer (runs under the disk operating systems MS-DOS or PC-DOS).

5. Why is AL important to learn? • Provides opportunity to know more about the operations of the PC • Enable control of the PC – access to specific hardware features • Quicker, smaller and have larger capacities compared to other HL languages

6. Machine Instructions • A machine instruction is a binary code that has a special meaning for a computer’s CPU – it tells the computer to perform a task. • Each machine instruction is precisely defined when the CPU is constructed, and it is specified to that type of CPU. For example: • 00000100 Add a number to the AL register. • 10100011 Move the AX register to another register

7. Registers are high-speed storage locations inside the CPU which are used by nearly every instructions • They are identified by 2-letter names, such as AH, AL, AX and so on. • We refer to machine instructions using hexadecimal numbers because they take up less writing space.

8. Assembly Language Instructions • Mnemonic – a short alphabetic code that literally “assists the memory” in remembering CPU instruction • It may be an instruction or a directive • E.g. an instruction MOV (move) • E.g. a directive DB (definite byte, used to create memory variables) • An instruction may contain zero, one or two operands.

9. Intel 8086 Architecture • There are 14 internal registers in Intel 8086 • All of them are represented in 16-bit mode or equivalent to 4 digits in hexadecimal • They are grouped into several categories • For general-purpose registers, each of these is a combination of two 8-bit registers which are separately accessible as AL, BL, CL, DL (the "low'' bytes) and AH, BH, CH, and DH (the "high'' bytes) • For example, if AX contains the 16-bit number 1234h, then AL contains 34h and AH contains 12h

10. Registers Instruction Pointer PC / IP General-purpose registers Special-purpose registers segment registers Status register FLAGS AX = AH + AL, BX = BH + BL , CX = CH + CL, DX = DH + DL SP, BP, SI, DI CS, DS, ES, SS Figure 1: 14 Registers in Intel 8086

11. General Purpose Registers • There are 4 general-purpose registers, each of them is designed to play a particular role in common use: • AX – Accumulator Register • Used for operations involving i/o and most arithmetic such as MUL and DIV, require that one of the operands be in the accumulator • As a place to store data • BX – Base Register • The only general-purpose register which may be used for indirect addressing, MOV [BX], AX • As a place to store data • CX – Count Register • It may contain a value to control the number of times a loop is repeated or a value to shift/rotate bits left or right • As a place to store data • DX – Data Register • Used together with AX for the word-size MUL and DIV operations • As a place to store data

12. Special Purpose Registers • There are 4 special-purpose registers: • Stack Pointer Register (SP) • Holds add of top of stack • Base Pointer Register (BP) • Holds add of ref point in the stack • Source Index Register (SI) • Holds memory add of source • Destination Index Register (DI) • Holds memory add of destination

13. Segment registers • There 4 segment register: • Code Segment Register (CS) • Holds selector for code segment • Data Segment Register (DS) • Holds selector for data segment • Extra Segment Register (ES) • Holds selector for extra segment • Stack Segment Register (SS) • Holds selector for stack segment

14. Status Register • The status register, FLAGS, is a collection of 1-bit values, which reflect the current state of the processor and the results of recent operations • 9 of the 16 bits are used in the 8086. In this lesson you will only learn 6 flags: • Carry flag • Parity flag • Sign flag • Zero flag • Overflow flag • Auxiliary Flag

16. Instruction Pointer • The instruction pointer (sometimes called Program Counter - PC), IP, gives the address of the next instruction to be executed, relative to the code segment .

17. Debug commands

18. Example: 0100 MOV AX, 0123 Move value 0123H to AX 0103 ADD AX, 0025  Add value 0025H to AX 0106 MOV BX, AX  Move contents of AX to BX 0108 ADD BX, AX  Add contents of AX to BX 010A MOV CX, BX  Move contents of BX to CX 010C SUB CX, AX  Subtract contents of AX from CX 010E SUB AX, AX  Subtract AX from AX (clear AX) 0110 JMP 100  Go back to the start