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**********************************************************************************************
*
* Title: Star Fill (In Memory) Homework #1
*
* Objective: CMPEN472 Homework #1 demonstration program. Fill 225 bytes of memory with '*'.
*
* Revision: V1.0
*
* Date: Jan. 22, 2025
*
* Programmer: Jacob McDonnell
*
* Company: The Pennsylvania State Universtiy
* Electrical Engineering and Computer Science
*
* Algorithm: While Loop demo of HCS12 Assembly
*
* Register Use: A accumulator: character to be placed in memory
* B accumulator: counter of filled memory locations
* X register: pointer to memory location to fill
*
* Memory use: RAM locations $3000 to $30E1
*
* Input: No input, data and parameters are hard coded.
*
* Output: Memory locations $3000 to $30E1 are filled with '*' character.
*
* Observation: This program is designed for instruction purpose.
* This program can be a template for other loop based code.
*
* Comments: This program is developed and simulated using CodeWarrior Development Software.
*
**********************************************************************************************
* Parameter Declaration Section
*
* Export Symbols
xdef pgstart ; export 'pgstart' symbol
absentry pgstart ; for assembly entry point
* Symbols and Macros
porta equ $0000 ; i/o port address
portb equ $0001
ddra equ $0002
ddrb equ $0003
**********************************************************************************************
* Data Section
*
org $3000 ; reserved memory starting address
here ds.b $e1 ; 225 memory locations reserved
count dc.b $e1 ; constant counter for stars = 225
*
**********************************************************************************************
* Program Section
*
org $3100 ; Program Start Address in RAM
pgstart ldaa #'*' ; Load '*' into accumulator A
ldab count ; Load count of stars into B
ldx #here ; Load address pointer into X
loop staa 0,x ; Put a start at this memory location
inx ; Go to the next memory location
decb ; Decrement the counter
bne loop ; Loop if not done
done bra done ; Task finished
end ; Last line of a file
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