Commodore Plus 4 - Page 258

246 Machine Language on the Commodore Plus/4
2008
200A
200C
200F
2011
2013
2016
2018
201A'
201C
201E
2020
2022
2023
2025
2027
06
06
AD
29
05
8D
A5
29
F0
A5
29
D0
E8
E0
D0
00
FF
FF
15
0F
FF
15
A5
08
FA
A5
08
FA
08
DB
ASL
ASL
FF LDA
AND
ORA
FF STA
LDA
AND
BBQ
LDA
AND
BNE
INX
CPX
BNE
BRK
$FF
$FF
5FF15
#$0F
$FF
$FF15
$A5
#$08
$2016
$A5
#$08
$201C
#$08
$2002
for putting in the background color
register.
Get value in background register.
Clear the luminance bits.
Put in the new luniance information.
Put value in background register.
Get low byte of jiffy clock timer.
Look at bit 3.
If still zero, look again.
Get low byte of jiffy clock timer.
Look at bit 3.
If still one, look again.
Increment the current luminance.
See if finished.
If not, go on.
Stop execution.
Execute the program with G 2000.
BCC—Branch If Carry Flag Is Clear
This instruction examines the current status of the carry bit. If it is cleared to 0,
the branch occurs. If it is 1, the branch does not occur, and execution continues
with the following instruction.
Example:
Operation: Branch on C- 0
Addressing Mode Syntax
Flags Affected: None
Opcode Bytes Cycles
Relative
BCC Shhhh
90
2*
* Add 1 cycle when the branch is taken. Add 2 cycles when it is taken across a page boundary.
When adding a 1-byte number to a 2-byte number, you can use standard double
precision addition, but the operation may be speeded up by using a BCC to
determine when to increment the high byte. In this example, the 8-bit number
stored at $2100 is added to the 16-bit number stored at $2101 -$2102 (high byte to
low byte), and the result is stored in $2101-$2102.
Clear carry flag prior to addition.
Get single byte to add.
Add to low byte of $2101-$2102.
Store result.
If carry is clear, branch to finish.
Otherwise, increment the high byte.
Stop execution.
sooo
2001
2004
2007
200A
200C
200F
18
AD
6D
8D
90
EE
00
00
02
02
03
01
21
21
21
21
CLC
LDA
ADC
STA
BCC
INC
BRK
$2100
$2102
$2102
$200P
$2101