Lecture 1 Assembly Language Programming

1. Lecture 1Assembly Language Programming Presented By Dr. Rajesh Palit Asst. Professor, EECS, NSU Originally Prepared By Dr. ShazzadHosain, EECS, NSU

2. What is Microcomputer? Interface Circuitry Central Processing Unit BUS Interface Unit Execution Unit Control Unit Basic Blocks of a microcomputer

3. Von-Neumann Concept/Model

4. Von-Neumann cycle • The Von-Neumann concept is the basic concept for universal microprocessors. The ix86 architecture is based on that concept.  • It means a stored-program computer in which an instruction fetch and a data operation cannot occur at the same time because they share a common bus. • It comprises a Control unit, an Execution unit, Memory, I/O unit • Instructions are treated in 5 cycles – (1) Fetch, (2) Decode, (3) Fetch Operands, (4) Execute, and (5) Update instruction Counter

5. Simplified Block Diagram of CPU

6. Registers • General Purpose Registers • AX, BX, CX, DX • Base Pointer (BP), Stack Pointer (SP) • Source Index (SI), Destination Index (DI) • Segment Registers • Code Segment (CS), Data Segment (DS), Stack Segment (SS), Extra Segment (ES) • Status Flag Register (FLAGs) • Instruction Pointer (IP)

7. Registers 16 bit Segment registers

8. Decimal, Binary and Hexadecimal Numbers • 23910= 11 1101 01112 = 3D716 • 239d = 1111010111b = 3D7h (d,b,h upper or lower case) • If no suffix is given, decimal is assumed • If a hexadecimal starts with a letter, an extra zero must be put in the beginning • Signed Numbers – 2’s complements form

9. Example Data • If AX = 20A2H then AH = 20H, AL = A2H • In other words, if AH = 1CH and AL = A2H then AX = 1CA2H AH AL 0010 0000 1010 0010 AX

10. The FLAGS register • FLAGS indicate the condition of the MP • Also control the operations • FLAGS are upward compatible from 8086/8088 to Pentium/Pentium Pro Figure 2.2: The EFLAG and FLAG registers

11. The FLAGs • Carry Flag – C • C = 1 if there is a carry out from the msb on addition • Or, there is a borrow into the msb on subtraction • Otherwise C = 0 • C flag is also affected by shift and rotate instructions 10101010 11101010 111010100 C = 1, in this case

12. The FLAGs • Parity Flag – P • P = 1 for even parity, if number contains even number of ones • P = 0 for odd parity, if odd number of ones 10101010 10101011 Even number of ones Odd number of ones P = 0 P = 1 Definition changes from microprocessor to microprocessor

13. The FLAGs • Zero Flag – Z • Z = 1 for zero result • Z = 0 for non-zero result • Sign Flag – S • S = 1 if MSB of a result is 1, means negative number • S = 0 if MSB of a result is 0, means positive number

14. The FLAGs • Trap Flag – T • Enables trapping through an on-chip debugging feature • T = 1 MP interrupts the flow of a program, i.e. debug mode is enabled • T = 0 debug mode is disabled • Direction Flag – D • Selects increment/decrement mode of SI and/or DI registers during string instructions • D = 1, decrement mode, STD(set direction) instruction used • D = 0, increment mode, CLD(clear direction) instruction used

15. The FLAGs • Overflow Flag – O • O = 1 if signed overflow occurred • O = 0 otherwise • Overflow is associated with the fact of range of numbers represented in a computer • 8 bit unsigned number range (0 to 255) • 8 bit signed number range (-128 to 127) • 16 bit unsigned number range (0 to 65535) • 16 bit signed number range (-32768 to 32767)

16. Signed vs. Unsigned Overflow • Let, AX = FFFFh, BX = 0001h and execute • ADD AX, BX 1111 1111 1111 1111 + 0000 0000 0000 0001 1 0000 0000 0000 0000 AX BX • Unsigned interpretation • Correct answer is 10000h = 65536 • But this is out of range. • 1 is carried out of MSB, AX = 0000h, which is wrong • Unsigned overflow occurred • Signed interpretation • FFFFh = -1, 0001h = 1, summation is -1+1 = 0 • Signed overflow did not occur

17. How instructions affect the flags? • Every time the processor executes a instruction, the flags are altered to reflect the result • Let us take the following flags and instructions • None • All • All except C • All (C = 1 unless result is 0, O = 1, 80H, or 8000H) • Sign Flag – S • Parity Flag – P • Zero Flag – Z • Carry Flag – C • MOV/XCHG • ADD/SUB • INC/DEC • NEG

18. Example 1 • Let AX = FFFFh, BX = FFFFh and execute ADD AX, BX FFFFh + FFFFh 1 FFFEh The result stored in AX is FFFEh = 1111 1111 1111 1110 S P Z C • = 1 because the MSB is 1 • = 0 because the are 15 of 1 bits, odd parity • = 0 because the result is non-zero • = 1 because there is a carry out of the MSB on addition

19. Example 2 • Let AX = 8000h, BX = 0001h and execute SUB AX, BX 8000h - 0001h 7FFFh The result stored in AX is 7FFFh = 0111 1111 1111 1111 S P Z C • = 0 because the MSB is 0 • = 0 because the are 15 of 1 bits, odd parity • = 0 because the result is non-zero • = 0 because there is no carry

20. An Assembly Program movax,4C00h int 21h main endp fact proc push bx movbl, al mov ax, 1 again: mulbl decbl jnz again pop bx ret fact endp end main Title Example .model small .data list db10,17,11,25,13 large db 0 count db 5 .code main proc mov ax, @data mov ds, ax mov al, 5 call fact

21. An Assembly Program #include <stdio.h> int main (void) { inti, j ; ********* // comment ********* } Example 3-5 of Barry B. Brey’s book

22. An Assembly Program Cont. • What is the content of BX? AH AL 00h 10h AAhAAh AX BH BL BX 10h 00h AAh AAh 00h 00h DATA1 DATA2 DATA3 DATA4

23. Assembly Language Structure

24. An Assembly Program • SMALL model allows one data segment and one code segment • TINY model directs the assembler to assemble the program into a single segment • DB for Define Byte (one single byte) • DW for Define Word (two consecutive bytes) 10h 00h AAh AAh 00h 00h DATA1 DATA2 DATA3 DATA4

25. Another Example

26. References • Ch 6, Digital Logic and Microcomputer Design – by M. Rafiquzzaman • Ch 2, Intel Microprocessors – by Brey • Ch 5, Assembly Language Programming – by Charles Marut