Commodore Plus 4 - Page 297

The 6803 Microprocessor 285
TAY—Transfer Value in Accumulator into Y Register
The value currently in the accumulator is placed in the Y register.
Operation: A -+ Y Flags Affected: N, Z
Addressing Mode Syntax Opcode Bytes Cycles
Implied TAY A8 1 2
The value in the accumulator is not altered. If the value transferred is negative,
the N flag is set to 1; otherwise it is cleared to 0. If the value transferred is zero, the
Z flag is set to 1; otherwise it is cleared to 0.
Example: In the following program a data byte must be multiplied by 2 to be used as an
index. Since shifts may not be performed on the Y register, the shift left is
performed in the accumulator, and the result is copied into the Y register using
the TAY instruction.
. 2000 AD 00 21 LDA $2100 Get the data byte.
. 2003 OA ASL Multiply by 2.
• 2004 A8 TAY Transfer the index value into .Y.
. 2005 B9 00 22 LDA $2200,Y Load the referenced data.
. 2008 00 BRK Stop processing.
TSX—Transfer Value in Stack Pointer into X Register
The value currently in the stack pointer is placed in the X register.
Operation: SP-* X Flags Affected: N, Z
Addressing Mode Syntax Opcode Bytes Cycles
Implied TSX BA 1 2
The value in the stack pointer is not altered. If the value transferred is negative,
the N flag is set to 1; otherwise it is cleared to 0. If the value transferred is zero, the
Z flag is set to 1; otherwise it is cleared to 0.
Example: This program illustrates the functioning of the stack. The TSX instruction is used
to copy the stack pointer into the X register. The value of the stack pointer when
the program begins executing is stored at $2100. Then the accumulator is pushed
onto the stack, and the resulting stack pointer is stored at $2101. The accumulator
is then pulled from the stack, and the new stack pointer is stored at $2102.