Commodore Plus 4 - Page 299

The 6502 Microprocessor 387
TXS—Transfer Value in X Register into Stack Pointer
The value currently in the X register is placed in the stack pointer.
Operation: X^ SP Flags Affected: None
Addressing Mode Syntax Opcode Bytes Cycles
Implied TXS 9A 1 2
The value in the X register is not altered. The information stored in the stack
area (page 1) is not changed by this instruction. Only which byte the stack pointer
points to is changed.
Example: When the computer is reset, part of the process is to reset the stack pointer to its
initial value, $FF. This example illustrates how that would be done using the TXS
instruction.
. sooo
. 2002
. 2003
A2 PP
9A
00
LDX
TXS
BKK
#$PP
Load .X with the
value.
Transfer to SP.
Stop processing.
desired pointer
TYA—Transfer Value in Y Register into Acciunulator
The value currently in the Y register is placed in the accumulator.
Operation: Y-+A Flags Affected: Nf Z
Addressing Mode Syntax Opcode Bytes Cycles
Implied TYA 98 I 2
The value in the Y register 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 finds the offset from $8020 of the first location containing the value
SEA in $8020-$811F by incrementing the Y register. Then the offset from $8000 is
calculated using the TYA instruction to transfer the contents of the Y register to
the accumulator and adding. Note that the status of the carry bit at $2010
indicates whether the first $EA occurs in $8020-$80FF (carry clear) or
$8100-$811F (carry set).
. 2000 A0 00 LDY #100 Start .Y at zero.
. 2002 A9 EA LDA #$EA Load the accumulator with the byte
to search for.