Commodore Plus 4 - Page 287

The 6502 Microprocessor 278
The stack pointer is incremented, and the value it points to is placed in the
processor status register.
Example: This example shows BASIC setting up for a SYS command. The value to send in
the status register is stored in S07F5.
. A7BE AD FS 07 LDA $07F5 Get the status register value from
memory.
. A7C1 48 PHA Push the value onto the stack.
. A7C2 AD F2 07 LDA $07F2 Set up .A.
. A7C5 AE F3 07 LDX $07F3 Set up .X.
. A7C8 AC F4 07 LDY $07F4 Set up .Y.
. A7CB 2B PLP Pull the status register from the
stack.
. A7CC 6C 14 00 JMP ($0014) Jump to the user routine.
R0L—Rotate Left One Bit
Each bit in the specified operand is rotated one bit to the left. The high bit is
rotated into the carry flag, and the carry flag is rotated into the low bit.
Operation: C*-[7
Addressing Mode
Accumulator
Zero page
Zero page, X
Absolute
Absolute, X
Syntax
ROL
ROL
$hh
ROL $hh,X
ROLShhhh
ROL $hhhh,X
Flags Affected: N, Z, C
Opcode
2A
26
36
2E
3E
Bytes
1
2
2
3
3
Cycles
2
5
6
6
7
Bit 7 of the operand is rotated into the carry flag. Bits 6 through 0 are rotated
into bits 7 through 1, respectively. The carry flag is rotated into bit 0. If bit 7 of the
result is set to 1, the N flag is set to 1; otherwise it is cleared to 0. If the result is
zero, the Z bit is set to 1; otherwise it is cleared to 0.
Example: This program is a 4-byte shift left (or multiplication by 2). If an overflow occurs,
the border color is incremented. The ROL instruction is used to shift left and
transmit the information between bytes by means of the carry bit.
• 2000 OE 03 21 ASL $2103 Shift the low byte to the left.
. 2003 2E 02 21 ROL $2102 Rotate the next byte to the left.
. 2006 2E 01 21 ROL $2101 Rotate the next byte to the left.