Memory Map
| Start | End | Description |
|---|---|---|
| 0000 | 001F | Internal registers |
| 0020 | 003F | I/O select |
| 0040 | 007F | RTC registers + RAM |
| 0080 | 3FFF | RAM |
| 4000 | 5FFF | Used by expansion unit |
| 6000 | 7FFF | ROM #4 (Option ROM) |
| 8000 | 9FFF | ROM #3 |
| A000 | BFFF | ROM #2 |
| C000 | DFFF | ROM #1 |
| E000 | FFFF | ROM #0 |
ROM #0 and #1 are known as the I/O ROMs, handling system reset and providing functions for using the LCD, keyboard, clock, printer, speaker, serial communication, etc. The I/O ROMs are equivalent to the BIOS in modern PCs. ROM #0 also contains the interrupt vector table at FFF0-FFFF. FFFE-FFFF determines what the program counter should be set to on power up or reset. In the standard set of ROMs for the HX-20, this value is E000, the start of ROM #0.
ROM #2 and #3 contains the BASIC interpreter. If the BASIC ROMs are removed from the motherboard, the BASIC option in the main menu will disappear, leaving only MONITOR. This is because ROM #3 contains a program header which is detected by the menu routines. This works the same for all user-created programs, except the program type is different.
The Expansion unit added up to 16 kByte of RAM and two ROM sockets. The latter could only be used by switching off the internal BASIC ROMS.
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