Epson HX-20 Memory Map

The HX-20 memory map is quite complicated/interesting. The basic machine has 32 kByte ROM and 16 kByte RAM, another ROM socket is available for a 8 kByte ROM. But as the Expansion Unit can contain 16 kByte RAM and two ROM sockets 16 kByte each, the 64 kByte addressing space is quite overloaded.

Main CPU address map

The Expansion Unit adds six different configurations to the Standard memory map.
Address Standard 2 x 16 kByte ROM 8 + 16 kByte ROM, 8 kByte RAM 1 x 16 kByte ROM, 16 kByte RAM 2 x 8 kByte ROM, 16 kByte RAM 2 x 8 kByte ROM 2 x 8 kByte ROM, 8 kByte RAM
0000-007F CPU internal registers or Clock RAM (64 bytes) or LCD buffer (480 bytes) - - - - - -
0080-00FF CPU internal RAM - - - - - -
0100-3FFF HX-20 board RAM - - - - - -
4000-5FFF empty Ext. ROM 1 (13B) Ext. RAM (1B,2B,4B,8B) Ext. RAM (1B,2B,4B,8B) Ext. RAM (1B,2B,4B,8B) X Ext. RAM (1B,2B,4B,8B)
6000-7FFF HX-20 board option ROM 4 (11E) ROM 1 (13B) Ext. RAM (11B,10B,6B,7B) Ext. RAM (11B,10B,6B,7B) X X
8000-9FFF HX-20 board BASIC ROM 3 (12E) Ext. ROM 0 (14B) Ext. ROM 0 (14B) Ext. ROM 0 (14B) Ext. ROM 1 (14B) Ext. ROM 1 (14B) Ext. ROM 1 (14B)
A000-BFFF HX-20 board BASIC ROM 2 (13E) Ext. ROM 0 (13B) Ext. ROM 0 (13B) Ext. ROM 0 (13B)
C000-DFFF HX-20 board UTILITY ROM 1 (14E) - - - - - -
E000-FFFF HX-20 board MONITOR ROM 1 (15E) - - - - - -
Settings in Expansion Unit SW2-1 OFF OFF ON ON OFF OFF
SW2-2 OFF ON OFF OFF OFF ON
SW2-3 ON ON ON OFF OFF OFF
SW2-4 OFF OFF OFF ON ON ON
J1 A A A B B B
J2 A A A B B B

Source: HX-20 Technical Manual, H8290013-1, P. 3-24, 3-30, 4-34 and Technical Support Document number 72a

Legend:
- same as Standard
X not usable
(..)board positons

So, to keep it short:

All this is set in hardware, so no software switching.

Last update: 2009-12-05

fjkraan@xs4all.nl