Commodore Plus 4 - Page 272

260 Machine Language on the Commodore Plus/4
. SOOD FO 08 BEQ $8017 If the desired value is found, quit.
. SOOP C6 D8 DEC $D8 Decrement the low byte of the
indirect address by 1.
.2011 DO F8 BITE $200B If not zero, go back.
. 2013 C6 D9 DEC $D9 When the low byte is zero, decrement
the high byte.
. 2015 DO F4 BITE $200B If not zero, go back.
. 2017 00 BRK Stop processing.
The address of the last occurrence of an $EA is stored in $D8-$D9 following
the execution of this program.
DEX—Decrement the X Register by 1
The value currently in the X register is decremented by 1.
Operation: X - 1 — X Flags Affected: N, Z
Addressing Mode Syntax Opcode Bytes Cycles
Implied DEX CA 1 2
The value stored in the X register is decreased by 1. If the result is negative, the
N flag is set to 1; otherwise it is cleared to 0. If the result is zero, the Z flag is set to
1; otherwise it is cleared to 0.
Example: This program copies contents of zero page into $2100-$21FF. The X register is
used as the index. The DEX instruction is used to update the index value.
. 2000 A2 00 LDX #$00 Initialize .X to zero.
. 2002 B5 00 LDA $00,X Get a value from zero page.
. 2004 9D 00 21 STA $2100,X Store the value in $2100-$2IFF.
. 2007 CA DEX Decrement the index by 1.
. 2008 DO F8 BWE $2002 If not equal to zero, go back.
. 200A 00 BRK Stop processing.
The operating system uses many zero page locations, so using an M (Memory)
command to examine zero page from the monitor may not reveal the contents of
zero page while a program is executing. A routine similar to the preceding one can
be used in a program to transfer the contents of zero page to a safe location prior
to returning to the monitor.