Commodore Plus 4 - Page 260

248 Machine Language on the Commodore Plus/4
Operation: Branch on Z- 1 Flags Affected: None
Addressing Mode Syntax Opcode Bytes Cycles
Relative BEQ $hhhh FO 2 2*
* Add 1 cycle when the branch is taken. Add 2 cycles when it is taken across a page boundary.
Example: In this example, the BEQ instruction is used to cause the program to continue
looping until a key is hit.
. S000 20 E4 FP JSR $FFE4 Call the ROM subroutine to look for
input.
. 2003 FO FB BEQ $2000 If zero is returned, then there is no
input, so look again.
. 2005 00 BRK Stop processing.
BIT—Test Bits in Memory with Accumulator
Bits 6 and 7 of the operand are transferred to the V and N flags. The values in the
accumulator and the operand are logically ANDed, and a zero result is indicated
in the Z flag. Neither the value in the accumulator nor in the operand is altered.
Operation: A AM Flags Affected: N, Z, V
M7-N
M6-V
Addressing Mode Syntax Opcode Bytes Cycles
Zero page BIT $hh 24 2 3
Absolute BITShhhh 2C 3 4
If bit 7 is set to 1 in the specified memory location, then the N flag is set to 1;
otherwise it is cleared to 0. If bit 6 is set to 1 in the specified memory location, then
the V flag is set to 1; otherwise it is cleared to 0. If the result of ANDing the value
in the accumulator with the value in the specified memory location is 0, the Z flag
is set to 1; otherwise it is cleared to 0. The values stored in the accumulator and the
memory location are not changed.
Example: The BIT instruction is particularly useful when information is required about
some memory location, but the value in the accumulator must be preserved. The
example counts the number of bits set to 1 in the value stored at $2100. The
accumulator keeps track of which bit it is on.
. 2000 A2 00 LDX #$00 .X will contain the bit count.