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SYSTEMS PROGRAMMING CHAPTER 2 PROGRAMMING IN ASSEMBLY LANGUAGE Er. Bharadwaj Choudhury HIGH LEVEL LANGUAGE CONSTRUCTS Constants: Numerical, string or other quantities whose unchanging actual values are known when the program is written Variables: Name given to a value. The value may change when the program is executed Reserve words: if, then, else, return, float, real Procedures: Functions or subroutines, which may or may not have arguments and may or may not return values ASSEMBLY LEVEL LANGUAGE CONSTRUCTS Computer’s memory: A sequence of bytes, words, blocks. Each computer have a fixed amount of working memory CPU’s registers: Each computer have a set of working as well as special registers Instructions: Each assembly language must have a set of instructions based on the microprocessor used in the computer, along with additional instructions available in the assembler Location Counter: Its task is to keep track of the current location of the instruction being processed relative to the beginning of the next group of instructions HOW TO RUN ASSEMBLY PROGRAMS ? Step 1 : Enter the assembly language program through a text editor or an inbuilt editor of the assembler. The inbuilt editor is known as work bench program Step 2 : The assembly program file or the source file is a ASCII file having the extension .ASM Step 3 : The assembler accepts this source file and converts it into an object file .OBJ in machine language. The linker converts such object file(s) into some executable form, such as .EXE, .BIN, etc through different passes. Sometimes the extension of inclusion file is .INC SOME COMMERCIAL ASSEMBLERS Borland turbo assembler (TASM) : This is a MS-DOS based assembler. Suppose MYTASM.ASM is a source program in TASM syntax, then the command C:\TASM>TASM MYTASM assemblers the source program and produces MYTASM.OBJ, provided the assembly program is correct. Microsoft assembler (MASM) : The name has traditionally been referred to the Microsoft Macro assembler. The software for MASM contains an executable program file ML.EXE. Here, ML assembles and links one or more assembly language source program files, producing a .OBJ file and an executable .EXE file SOME COMMERCIAL ASSEMBLERS MicroAsm : It is a windows-based Integrated Development Environment(IDE) for the assembly programmer. This includes a source editor and an Intel 8086 assembler with easy syntax Netwide Assembler(NASM) : It is a 80x86 processorbased assembler. It supports a wide variety of object file formats such as a.out, ELF, COFF, Win32, etc. Simple binary files are also generated by this assembler. This assembler works under different operating environments such as MS-DOS, WINDOWS and LINUX ASSEMBLER TYPES One pass assemblers Two pass assemblers One and half pass assemblers Multi-pass assemblers ASSEMBLY LANGUAGE FORMATS Different assembly language instruction formats differ on details like pseudo operations used, length of identifiers, formats of a constant, etc However, most assembly language instructions has 2 mandatory and 2 optional fields. [Label] Opcode Operand [Comments] Label : a symbol of the assembly program. Can store maximum of 6-8 characters Opcode : contains mnemonic code for machine or pseudo operations Operand : specifies 0 or more operands, separated by commas Comments : high-level description of the program HOW ASSEMBLY PROGRAMS ARE EXECUTED? Consider the assembly program shown below [to be executed in Intel 8086 machine]: Stm. No. Label 1 For your understanding Opcode Operand 0RG 100h 2 NUM1 DW 20 3 NUM2 DW 13 4 SUM DW ? 5 MOV AX, NUM1 6 ADD AX, NUM2 7 MOV SUM,AX 8 END Red color indicates machine operations, Blue color indicates pseudo operations MOTAB & POTAB Mnemonic Opcode Length(byte) MOV AX,memo B8 3 ADD AX,memo 02 3 MOV memo,AX A2 3 Pseudo operation Task ORG Start the program and initialize LC with the value of operand, else initialize it to 0 DW Define the constant and word storage requirement. Update LC value END End indicates the end of program. Also, enables allocation of addresses for the unallocated literals in the literal table. Control send to the next pass PASS 1 OF THE ASSEMBLY PROCESS Stm. No. 1 : LC ← 100h Stm. No. 2 : NUM1 in symbol table with value 100h LC ← 100h + 2h = 102h Stm. No. 3 : NUM2 in symbol table with value 102h LC ← 102h + 2h = 104h Stm. No. 4 : SUM in symbol table with value 104h LC ← 104h + 2h = 106h Stm. No. 5: LC ← LC + 3h =109h Stm. No. 6 : LC ← LC + 3h =10Ch Stm. No. 7 : LC ← LC + 3h =10Fh Stm. No. 8: Opcode is END, indicating end of program PASS 2 OF THE ASSEMBLY PROCESS Stm. No. 1 : Allocate the LC ← 100h Stm. No. 2-4 : Already processed in pass 1 and symbols stored in symbol table Stm. No. 5: Machine code= B8 + Address of NUM1 = B80100 Stm. No. 6 : Machine code= 02 + Address of NUM2 = 020102 Stm. No. 7 : Machine code= A2 + Address of SUM = A20104 Stm. No. 8: Opcode is END, indicating termination of program SUMMARY : THE TWO PASSES Pass I is to construct a symbol table and a literal table using the MOTab and POTab Pass I keeps a copy of the user’s program for Pass II An intermediate mnemonic program is output of Pass I The task of Pass II is to generate the object code from the copy of the user’s program in Pass I Pass II provides the linker & loader with the necessary information for linking object code with link libraries and ultimately loading the executable code in memory for execution PRACTICE PROBLEM Generate symbol table (Pass 1) and machine code(Pass 2) for the assembly program below running on a SIC machine [Note: SIC is a 24-bit machine] Stm. No. Label Opcode Operand 1 SIC START 02 2 LDA NUM1 3 ADD NUM2 4 STA SUM 5 NUM1 WORD 20 6 NUM2 WORD 13 7 SUM WORD 0 8 END Red color indicates machine operations, Blue color indicates pseudo operations MOTAB & POTAB FOR PRACTICE PROBLEM Mnemonic Opcode Length(byte) LDA m 00 3 ADD m 18 3 STA m 0C 3 Pseudo operation Task START Start the program and initialize LC with the value of operand, else initialize it to 0 WORD Define the constant and storage requirement. Update LC value END End indicates the end of program. Also, enables allocation of addresses for the unallocated literals in the literal table. Control send to the next pass THANK YOU