Commodore Plus 4 - Page 282

270 Machine Language on the Commodore Plus/4
. 2008 85 D8 STA $D8 Store in the low byte of the
originating address.
. SOOA A9 28 LDA #$28 Load .A with the low byte of the
second line address.
. 200C 85 DA STA $DA Store in the low byte of the
destination address.
. 200E AO 27 LDY #$27 Initialize .Y to move one line.
. 2010 Bl D8 LDA ($D8),Y Get the character from the first line.
. 2012 91 DA STA ($DA),Y Put on the second line.
. 2014 88 DEY Decrement the index register.
. 2015 10 F9 BPL $2010 If not done, go on.
. 2017 00 BRK Stop processing.
If the screen scrolls upon execution of the BRK instruction, the effect of the
program will not be seen. To avoid this, clear the screen before executing the
program.
LSR—Shift Right One Bit
Each bit in the specified operand is shifted one bit to the right. The low bit is
shifted into the carry flag, and a 0 is shifted into the high bit.
Operation: 0 — [7 0]-+C Flags Affected: N, Z, C,
Addressing Mode Syntax Opcode Bytes Cycles
Accumulator
Zero page
Zero page, X
Absolute
Absolute, X
LSR
LSR $hh
LSR $hh,X
LSR $hhhh
LSR $hhhh,X
4A
46
56
4E
5E
1
2
2
3
3
2
5
6
6
7
Bit 0 of the operand is shifted into the carry flag. Bits 7 through 1 are shifted
into bits 6 through 0, respectively. Since a 0 is shifted into the high bit, the result
can never be negative. Hence, the N flag is always cleared to 0. If the result is zero,
the Z bit is set to 1; otherwise it is cleared to 0.
Example: This program counts the number of bits set to 1 in memory location $2100. The
LSR instruction is used to shift the bits one by one into the carry bit for testing.
. 2000 AO 07 LDY #$07 .Y counts the 8 bits.
. 2002 A2 00 LDX #$00 .X counts the number of bits set to 1.
. 2004 AD 00 21 LDA $2100 Get the byte to count.
. 2007 4A LSR Shift bit 0 into the carry.