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Segments and Pseudo Operations

Segments and Pseudo Operations. Program Development. Format of the source code. Each line of code is divided into fields: Label Field Operation Field Operand Field Comment Field. Code and Data Segments. We can organize the code into blocks called segments

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Segments and Pseudo Operations

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  1. Segments and Pseudo Operations Program Development

  2. Format of the source code • Each line of code is divided into fields: • Label Field • Operation Field • Operand Field • Comment Field

  3. Code and Data Segments • We can organize the code into blocks called segments • Segments specify sections of code, data, and reserved areas of memory • Segments are useful for modularized program development

  4. Segments

  5. Pseudo-Instructions (1) • Pseudo-instructions are directives for the Assembler • We have already seen some of them • ORG - defines the absolute address of a segment ORG $9000 ;set origin of text segment to $9000 • EQU - defines an equivalent symbol for a value ONE EQU $01 PORTB EQU $1004 • END – delimits the end of the assembly • RMB – stands for “reserve memory byte(s)”. It allocates a specified number of bytes. varname RMB 2 • DS – stands for “define space”. It is the same as RMB varname DS 2

  6. Pseudo Instructions (2) • FCB – stands for “form constant byte(s)”. It allocates byte(s) of storage with initialized values addrfirstval FCB $01,250,@373,%111001101 FCB $23 • DB – stands for define byte(s). It is the same as FCB • FDB – stands for “form double byte(s)”. It is a 16-bit version of FCB FDB $0001,$1234,$FFFA • DW – stands for “define word”. It is the same as FDB • FCC – stands for “form constant character(s)” and it is used to allocate and initialize memory for storage of a string addrfirstchar FCC “some string” FCC “Alarm 5A high!”

  7. Example * program to drive a stepper motor size equ 4 PORTB equ $1004 ;PB3-PB0 to stepper org $9000 main ldaa #size ldx #steps ;address at which $05 is located step ldab 0,x inx stab PORTB ; step the motor deca bne step bra main steps fcb 5,6,10,9 ;output sequence org $FFFE fdb main end

  8. Pseudo Instructions (3) • FILL – sets a number of bytes to a specified value FILL $FF 16 • ZMB – stands for “Zero Memory Bytes” and initialize a specified number of memory bytes to zero ZMB 16 • BSZ – stands for “Block Store Zeros and it the same as ZMB BSZ 16

  9. Assembly two-pass Process • For an assembler to understand labels and symbols, it must work through the source code twice . It follows as a two-pass process • After the first step the assembler build a “symbol table” that gives the address of each label and symbol.

  10. Assembler options and preprocessor directives • Assembler options and preprocessor directives are assembler specific • Assembler options must occur at the beginning of the source code, and start in column one of the code with a $ sign. • The preprocessor directives follows the assembler options and begin with a %. The preprocessor directives tell the assembler to do something before beginning the assembly process%INCLUDE “d:\include\iolib.h”

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