IOBASE
| Function | Address | Description |
|---|---|---|
| IOBASE | FFF3 | Define I/O memory page |
This routine sets the X and Y registers to the address of the memory section where the memory mapped 110 devices are located. This address can then be used with an offset to access the memory mapped I/O devices in the Commodore 64. The offset is the number of locations from the beginning of the page on which the I/O register you want is located. The X register contains the low order address byte, while the Y register contains the high order address byte.
This routine exists to provide compatibility between the Commodore 64, VIC-20, and potential future models of the Commodore 64. If the I/O locations for a machine language program are set by a call to this routine, they should still remain compatible with future versions of the Commodore 64, the KERNAL and BASIC.
Example:
Note: FREKZP (0x00FB) in this example is a 4 byte block of zeropage memory unused by Basic or Kernal and is available for user code.
For the C64 the value returned is 0xDC00 - address of CLA #1
For the C128 the value returned is 0xD000 - address of the VIC chip