Commodore Plus 4 - Page 265

The 6503 Microprocessor 253
Operation: Branch onV-1 Flags Affected: None
Addressing Mode Syntax Opcode Bytes Cycles
Relative BVS $hhhh 70 2 2*
* Add 1 cycle when the branch is taken. Add 2 cycles when it is taken across a page boundary.
Example: This example uses the BIT instruction to transfer the status of bit 6 of the low byte
of the jiffy clock to the overflow flag. First, the program waits until this bit is
clear, then until it is set. An increment of the border color signals the end of the
wait.
. 2000 24 A5 BIT $A5 Transfer bit 6 of jiffy clock to over
flow flag.
. 2002 70 PC BVS $2000 Continue waiting if the bit is set.
. 2004 EE 19 PF LKTC $FF19 Increment the border color.
. 2007 24 A5 BIT $A5 Transfer bit 6 of jiffy clock to over
flow flag.
. 2009 50 PC BVC $2007 Continue waiting if the bit is clear.
. 200B EE 19 PP IUC $PP19 Increment the border color.
. 200E 00 BRK Stop processing.
CLC—Clear the Carry Flag
The carry flag is cleared to 0.
Operation: 0 — C Flags Affected: C
Addressing Mode Syntax Opcode Bytes Cycles
Implied CLC 18 1 2
Example: This program adds the contents of $2100 to the contents of $2101 and stores the
result in $2102. Because the ADC instruction adds the value of the carry to the
sum of the accumulator and the operand, the CLC instruction is used to clear the
carry flag before the addition. This ensures that the result is exactly the first value
plus the second value.
. 2000 18 CLC Clear the carry flag to prepare to add.
. 2001 AD 00 21 LDA $2100 Get the first value.
. 2004 6D 01 21 ADC $2101 Add the second value.
. 2007 8D 02 21 STA $2102 Store the result.
. 200A 00 BRK Stop processing.
Use the M (Memory) command to examine and change the contents of
$2100-$2102.