Commodore Plus 4 - Page 311

Using Interrupts on the Plus/4 899
Raster Interrupts
The Plus/4 is capable of interrupting the processor in response to the vertical
position of the raster beam on the TV or monitor. This capability can be used, for
example, to update information for the screen display while it is not seen, thus
eliminating flicker.
To enable raster interrupts, bit 1 of the interrupt enable register ($FF0A) must
be set to 1. The low 8 bits of the raster value at which the interrupt is to occur (a
9-bit number) must be placed in the raster compare register ($FF0B). The high bit
is the low bit of the interrupt enable register ($FF0A) and can be set to the
appropriate value at the same time as the enabling. The raster is on the usable
screen (not the border) from about $04 through $CB.
When a raster interrupt occurs, bit 1 of the raster interrupt status register
($FF09) is set to 1. Also, bit 7 of this register is set to indicate that an interrupt
occurred.
Example: In this example program, raster interrupts are used to create a band of changed
background color on the screen. First, an interrupt occurs near the top of the
screen. The processing of this interrupt includes decrementing the background
color and setting up the next interrupt. The second interrupt occurs a few lines
lower. The processing of this interrupt includes incrementing the background
color back to its original value and setting up the first interrupt again. This
process is repeated forever.
Disable interrupts for set up.
Low byte of address of service routine.
Store in low byte of IRQ vector.
High byte of address of service routine.
Store in high byte of IRQ vector.
Bit 1 set to 1, bit 0 cleared to 0.
Enable raster interrupts and clear high
bit of compare value.
Compare value is $30.
Store in low bits of compare value.
Start interrupts going.
Infinite loop.
Get interrupt status register.
Clear all bits that were set.
Get raster compare value.
Compare to $50 (the bottom interrupt).
If less than $50 (the top interrupt),
branch.
Cortpare value for top interrupt.
Store in compare value.
Increment the screen color.
2000
2001
2003
2006
2008
200B
200D
2010
2012
2015
2016
2019
201C
201F
2022
2024
2026
2028
202B
78
A9
8D
A9
8D
A9
8D
A9
8D
58
4C
AD
8D
AD
C9
90
A9
8D
EE
19
14
20
15
02
0A
30
0B
16
09
09
0B
50
0B
30
0B
15
03
03
FF
FF
20
FF
FF
FF
FF
FF
SEI
LDA
STA
LDA
STA
LDA
STA
LDA
STA
CLI
JMP
LDA
STA
LDA
CMP
BCC
LDA
STA
INC
#$19
$0314
#$20
$0315
#$02
$FF0A
#$30
$FF0B
$2016
$FF09
$FF09
$FF0B
#$50
$2031
#$30
$FF0B
$FF15