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MEMBOT

     Purpose:  Set bottom of memory
Call address:  $FF9C (hex)  65436 (decimal)
Communication registers:  X, Y
Preparatory routines:  None
Error returns:  None
Stack requirements:  None
Registers affected:  X, Y

Description:

This routine is used to set the bottom of the memory. If the accumulator carry bit is set when this routine is called, a pointer to the lowest byte of RAM is returned in the X and Y registers. On the unexpanded Commodore 64 the initial value of this pointer is $0800 (2048 in decimal). If the accumulator carry bit is clear (=0) when this routine is called, the values of the X and Y registers are transferred to the low and high bytes, respectively, of the pointer to the beginning of RAM.

How to Use:

TO READ THE BOTTOM OF RAM
  1. Set the carry.
  2. Call this routine.

TO SET THE BOTTOM OF MEMORY
  1. Clear the carry.
  2. Call this routine.

EXAMPLE:

     ;MOVE BOTTOM OF MEMORY UP 1 PAGE
     SEC         ;READ MEMORY BOTTOM
     JSR MEMBOT
     INY
     CLC         ;SET MEMORY BOTTOM TO NEW VALUE
     JSR MEMBOT
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