APPENDIX TWELVE



     This also means that the area from 49152 to 52991 ($C000 to
     $CEFF) isn't used--almost 4K of free RAM for player missiles,
     machine language routines, anything you need it for. Be care-
     ful not to run over into the interrupt handlers at 52992 ($CF00).

54019          D303           PBCTL
     The PORT B controller on the 400/800; not used since there isn't
     one on the XL/XE series.

54528-54783    D500-D5FF      ....
     Unused in both XL and 400/800 models. Any access read or
     write to this area enables the cartridge control line CCNTL as in
     the cartridge interface in the 400/800.

55296-57343    D800-DFFF      FP
     Floating-point package; although corrected, the entry point re-
     mains the same as in the 400/800. You now get an error if you
     try to get a LOG or LOG 10 of 0. This area becomes addressable
     by the device when the OS switches out ROM to perform I/O on
     a device connected to the expansion slot.

     There are several tables built into the FP package:

     Address      Table
     56909/DE4D   Power of 10 coefficients
     57202/DF72   Logarithm coefficients
     57262/DFAE   Arctangent coefficients (unused?)

     The OS switches the floating point out and switches in the par-
     allel bus interface (PBI) ROM when an external device attached
     through the bus is selected, switching it back when the I/O is
     completed. This means an external device can't use floating
     point or any software which does (such as BASIC).

     The first 26 bytes of the hardware ROM vector area (when OS
     ROM is deselected) are:

     Byte          Hex         Use
     55296/55297   D800/D801   ROM checksum LSB/MSB
                               (optional)
     55298         D802        ROM revision number
                               (optional)
     55299         D803        ID number (128; $80)
     55300         D804        Device type (optional)
     55301         D805        JMP instruction ($4C)
     55302/55303   D806/D807   I/O vector LSB/MSB
     55304         D808        JMP
     55305/55306   D809/D80A   Interrupt vector LSB/MSB
     55307         D80B        ID number (145; $91)