Modified by: Ahmed Abu-Hajar, Ph.D. Lecture #10 - DIGITAVID

Modified by: Ahmed Abu-Hajar, Ph.D.
Lecture #10

Outline
Subroutines
D-BUG12 Utility Tool

Subroutines
Subroutines: Separate, independent module of the
program
Performs a specific task
Shortens code, provide “reusable code”
Good subroutine:
Independence: does not depend on other codes
Registers: Stores/restores key registers upon
entrance/exit using PSH and PUL commands (Store Key
registers on the Stack)
Data and code independent: Local variables do not use
direct and extended addressing modes

Subroutines
How to Call a Subroutine?
• The main program exist with
sequential execution order.
Call Subroutine by changing PC
to the subroutine location
This is done by using Branching
(BSR or JSR)
BSR limited by (-128- 127}
JSR jumps anywhere
Before Branching Must store all
key registers including PC in
memory (or Stack)
Once the subroutine finished, PC
goes back to the next step.
RTS is used to return.
Every subroutine MUST have RTS.
Program (*.asm)
Main Program:
Call Subroutine 1
Subroutine 1
Return

Subroutine: Example
STACKTOP = $8000
_main::
LDS #STACKTOP ;initialize SP
JSR DOIT ;jump to subroutine
HERE: BRA HERE
DOIT: PSHA ;store key registers on stack
PSHB
: ;subroutine actions
PULB ;restore key register values
PULA
RTS ;return from subroutine

Subroutine:
Passing Parameters: Pass parameter to the
subroutine when the program calls that
subroutine
Two types of calls
Call by Value: pass the actual value of the
parameter to the subroutine
Call by Reference: pass the memory address which
contains the parameter’s address

Subroutine:
Example of call by Value
This Example Swaps the values of
two consecutive memory locations
Org $4000
BASE EQU $4000
Num_ONE FCB $23
NUM_TWO FCB $3F
ORG $4100
LDS #$8000
LDX #BASE
LDAA $0,X
LDAB $1,X
BSR SWAP
STAA NUM_ONE
STAB NUM_TWO
END
SWAP PUSA
PUSB
PULA
PULB
RTS
Note: The opcode EXG A, B would
swap A& B. We did it here to
show how to use subroutine

Subroutine:
Example of call by Reference
Copy the ten consecutive values from
location $5000 to $6000
FIRST EQU $5000
SECON EQU $6000
ORG $4000
NUM_DATA FCB $0A
ORG $4100
LDS #$8000
LDX #BASE
LDAA NUM_DATA
LDX #FIRST
LDY #SECOND
JSR COPY
END
COPY COMPA #S00
BEQ DONE
LDAB 1,X+
STAB 1,Y+
DECA
BRA COPY
DONE RTS

D-BUG12 Utility
Subroutines for HC12
D-BUG12: Contains built-in subroutines for to monitor and or
debug the program.
Stored within ROM of the controller
Allows the programmer to alter memory locations or modify
registers or debug the program.
Contains 18 utility subroutines which are listed as follows:
Routine vector Address Comment
Main() $FE00 ;Start the D-Bug12 monitor program
getchar() $FE02 ;Fetch Character form Keyboard
Putchar() $FE04 ;Display Character form on Screen
printf() $FE06 ;Display formatted string on screen
GetCmdLine() $FE08 ;Fetch a command from keyboard
Scanhex() $FE0A ;Convert Hexadecimal to integer
isXdigit() $FE0C ;Check for hexadecimal digit
toupper() $FE0E ;Converts lowercase to uppercase

D-BUG12 Utility
Subroutines for HC12
Continue: Utility subroutines
Routine vector Address Comment
isalpha() $FE10 ;Check for alphabet character
strlen() $FE12 ;Return the length of the string
Srtcpy() $FE14 ;copies a string to another string
out2hex() $FE16 ;Output a byte of a two hexadecimal
;number
Out4hex() $FE18 ;Output two byte of four hexadecimal ;
;Numbers
setUserVector() $FE1A ;set user-specified ISR routine address
IWriteEEByte() $FE1C ;write a byte to the on chip EEPROM
EraseEE() $FE1E ;Erase a block of on-chip EEPROM
ReadMem() $FE20 ;Read Data From Memory
WriteMem() $FE22 ;Write data to memory

D-BUG12 Utility
Subroutines for HC12
For D-BUG12 routines, the 68HC12 insists on using
stack to transfer parameter variables
Treat parameters as 16-bit values
Parameters must be pushed onto the stack
Starting with parameter n to 1
Load vector address of the subroutine in index
register X

D-BUG12 Utility: Example
Use out2hex() utility subroutine to display $45 on
the computer screen
Store $0045 to stack before calling subroutine
LDD #$45 ;value for display
PSHD
LDX #$FE16 ;16-bit addr of subr
JSR 0,X ;call the subroutine
PULX ;clean up the stack