Segments and
Pseudo Operations
Program Development
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Format of the source code
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Each line of code is divided into fields:
Label Field
 Operation Field
 Operand Field
 Comment Field
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Code and Data Segments
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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
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Segments
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Pseudo-Instructions (1)
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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
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Pseudo Instructions (2)
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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!”
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Example
* program to drive a stepper motor
size equ 4
PORTB equ $1004
org $9000
main
ldaa #size
ldx #steps
step
ldab 0,x
inx
stab PORTB
deca
bne step
bra main
steps
fcb 5,6,10,9
;PB3-PB0 to stepper
;address at which $05 is located
; step the motor
;output sequence
org $FFFE
fdb main
end
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Pseudo Instructions (3)
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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
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Assembly two-pass Process
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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.
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Assembler options and preprocessor
directives
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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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