MDFS::Info.Comp.BBC.SROMs.MegaROM/htm

Replacing the Master System ROM

The BBC Master system ROM is a single 128K ROM in a 28-pin package. It holds the MOS and sideways ROMs 9 to 15. ROMs 15 and 14 hold extra parts of the MOS, but ROMs 13 to 9 can be freely replaced with other sideways ROMs.

Unfortunately, it is not possible to get 28-pin 128K EPROMs, they come in 32-pin packages. However, you can build an IC carrier using a 32-pin IC socket and a 28-pin IC socket, wired together as shown in this diagram:
+---------------+ | 1--+ 32-+ | +-2 | 31-+ | 1---3 | 30 +-28 | 2---4 | 29---27 | 3---5 | 28---26 | 4---6 | 27---25 | 5---7 | 26---24 | 6---8 | 25---23 | 7---9 +--24 22-+ 8---10 | 23---21 9---11 | 22---20 10---12 | 21---19 11---13 | 20---18 12---14 | 19---17 13---15 | 18---16 14---16-+ 17---15

See also Andrew Banham's site.

ROM layout, MOS 3.20

 ROM     ROM   Memory
Address Number Address Contents
-------------------------------------------------
&00000         &C000   MOS 3.20
&04000    9    &8000   DFS 2.24 + SRAM 1.04
&07900    9    &B900   spare
&08000   10    &8000   ViewSheet B1.0
&0C000   11    &8000   EDIT 1.00
&0FFF8   11    &BFF8   spare
&10000   12    &8000   BASIC 4.00
&14000   13    &8000   ADFS 1.50
&18000   14    &8000   VIEW B3.0
&1BA00   14    &BA00   MOS code
&1C000   15    &8000   TERMINAL 1.20 + TUBE & MOS code
&1F8DF   15    &B8DF   spare
&1F900   15    &B900   Character set

All code between locations &4000 and &1BA00 in the system ROM (location &BA00 in ROM 14) can be completely replaced. Additional code can be put at the end of ROM 9 at location &B900 (&07900 in the system ROM).

This is the replacement system ROM I use: MOS321.rom

MOS   MOS 3.20
ROM F TERMINAL 01 with Y2Kfix
ROM E VIEW J3.0 - VIEW with extended keyboard input
ROM D Acorn ADFS 1.53 - ADFS with IDE drivers
ROM C BASIC 4.00
ROM B HADFS 5.40
ROM A ANFS 4.25
ROM 9 DFS 2.24+Mouse - DFS 2.24 with double density bugfix, SRAM 1.04, Mouse 1.00