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

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:

If you want to use External ROM 0, you lose BASIC
If you want the full 16 kByte External RAM, you lose both the internal option ROM and 16 kByte of your external ROM (one 16 kByte ROM or two 8 kByte ROMs are still possible)

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

Last update: 2009-12-05

fjkraan@electrickery.nl