Commodore Plus 4 - Page 330

318 Machine Language on the Commodore Plus/4
. 2014 D0 FA BNE $2010
. 2016 AA TAX
. 2017 E0 20 CPX #$20
. 2019 B0 IF BCS $203A
. 201B A0 08 LDY #$08
. 201D BD 00 FF LDA $FF00,X
. 2020 F0 15 BEQ $2037
. 2022 30 04 BMI $2028
. 2024
. 2025
. 2026
. 2028
. 202A
. 202C
. 202E
• 2030
. 2031
. 2033
• 2034
. 2035
. 2037
. 2039
. 203A
. 203C
. 203E
88
0A
D0
85
A9
A2
95
CA
10
18
98
69
85
60
A9
85
60
FA
62
00
03
63
FB
80
61
80
66
DEY
ASL
BNE
STA
LDA
LDX
STA
DEX
BPL
CLC
TYA
ADC
STA
RTS
LDA
STA
RTS
$2022
$62
#$00
#$03
$63,X
$202E
#$80
$61
#$80
$66
This is always taken.
Put converted register number in .X.
Compare to highest valid register number
(+1).
If greater than or equalr exit with
error.
This keeps track of multiplications by 2.
Get the register contents.
If zero, a zero must be returned.
When negative, no more muliplying
required.
Decrement multiplication index.
Multiply by two.
This is always taken.
Store the mantissa in floating point
accumulator.
Get a zero.
Load .X with size of rest of accumulator*
Fill rest of accumulator with zero.
Decrement position in accumulator.
If not done, go back.
Prepare to add.
Get multiplication index.
Add 128 to get exponent for accumulator.
Store in exponent.
Return.
Get value with sign bit set.
Put in sign register to indicate error.
Return.
The SYS Command
Example:
The SYS command transfers control from a BASIC program to a machine-
language subroutine. When the subroutine is completed with an RTS instruction,
control is returned to the BASIC program. When it is necessary to pass parame
ters to and from the machine-language subroutine, the values are normally stored
in memory with POKE commands and retrieved using PEEK.
Note: You can set the values of the accumulator, X register, Y register, and
status register on entry to your subroutine automatically by POKEing their
desired values into 2034-2037 ($07F2-$07F5). The values of the registers on exit
from your subroutine are returned in the same locations.
In this example, a machine code routine to fill an area of the screen with a given
character is called from a BASIC program using the SYS command. The BASIC
program is as follows:
10 PRI]SrTCHR$(147)//I]SrPIJTPILLCHAEACTER ";:GETKEYA$: