Commodore Plus 4 - Page 264

252 Machine Language on the Commodore Plus/4
. SOOA 00 BRK Execute a BRK instruction (and stop
processing).
. 200B EE 19 FF INC $FF19 Increment the border color.
. SOOE 4C 4C F4 JMP $F44C Then jump to normal BRK
processor.
After you execute this program (by typing G 2000), the execution of a BRK
instruction will increment the border color before proceeding normally. When
you are finished with this example, reset the computer by typing G FFF6 so as to
restore the BRK instruction vector to its default value.
BVC—Branch. If Overflow Flag Is Clear
This instruction examines the current status of the overflow flag 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.
Operation: Branch onV-0 Flags Affected: None
Addressing Mode Syntax Opcode Bytes _Cycles
Relative BVC $hhhh 50 2 2*
* Add 1 cycle when the branch is taken. Add 2 cycles when it is taken across a page boundary.
Example: This program adds the contents of $2100 to the contents of $2101 and stores the
result in $2102. If a two's complement overflow occurs, the border color is
incremented.
2000
S001
2004
2007
200A
200C
200F
18
AD
6D
8D
50
EE
00
00
01
02
03
19
21
21
21
FF
CLC
LDA
ADC
STA
BVC
IWC
BRK
$2100
$2101
$2102
I200F
$FF19
Prepare for addition.
Get the first value.
Add the second value.
Store the result.
Branch if no overflow occurred.
Increment the border color.
Stop processing.
Examine and change the contents of $2100-$2102 to experiment with two's
complement overflows.
BVS—Branch If Overflow Flag Is Set
This instruction examines the current status of the overflow flag bit. If it is set to
1, the branch occurs. If it is 0, the branch does not occur, and execution continues
with the following instruction.