Commodore Plus 4 - Page 405

The Joystick Ports 393
2
3
4
5
6
7
Left
Right
Unused
Unused
Fire for
Fire for
port
port
1
2
(unused
(unused
for
for
2)
1)
When a switch is engaged, the bit is cleared to 0; otherwise it is set to 1. All of the
unused bits are always set to 1. As a result, an $FF indicates a neutral position for
the joystick.
Example: This is a subroutine for reading joystick port 1. On return, carry clear means
FIRE; carry set is no FIRE. The X register is set to -1 ($FF), 0, or 1 to indicate
left, middle, and right positions, respectively, in the horizontal direction. The Y
register is set to -1 ($FF), 0, or 1 to indicate down, middle, and up positions,
respectively, in the vertical direction.
2000 A2 FA LDX #$FA
2002 78 SEI
2003 8E 08 FF STX $FF08
2006 AD 08 FF LDA $FF08
2009 8E 08 FF STX $FF08
200C
200F
2011
2012
2014
2016
2017
2019
201A
201B
201D
201E
201F
2021
2022
2023
2025
2026
2027
2028
2029
CD
D0
58
A2
A0
4A
B0
C8
4A
B0
88
4A
B0
CA
4A
B0
E8
4A
4A
4A
60
08
F2
00
00
01
01
01
01
FF CMP
BNE
CLI
LDX
LDY
LSR
BCS
INY
LSR
BCS
DEY
LSR
BCS
DEX
LSR
BCS
INX
LSR
LSR
LSR
RTS
$FF08
$2003
#$00
#$00
$201A
$201E
$2022
$2026
Latch value for port 1.
Disable interrupts.
Store latch value in keyboard latch
register.
Read keyboard latch register.
Store latch value in keyboard latch
register.
Compare to previous reading.
If not equal, do it again.
Enable interrupts.
Initially set changes to zero.
Look at bit 0.
If set, switch is off.
Up switch is on, increment y change.
Look at bit 1.
If set, switch is off.
Down switch is on, decrement y change.
Look at bit 2.
If set, switch is off.
Left switch is on, decrement x change.
Look at bit 3.
If set, switch is off.
Right switch is on, increment x change.
Skip bit 4.
Skip bit 5.
Carry reflects status of fire switch.
Return.
For port 2, the $FA in the instruction at $2000 must be changed to $FD and an
additional LSR instruction placed at $2029, with the RTS at $202A.