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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 |
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 setAll 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
ROM ROM Memory Address Number Address Contents ------------------------------------------------- &00000 &C000 MOS 3.50 &04000 9 &8000 DFS 2.45 &06E44 9 &AE44 spare &06F00 9 &AF00 MOS code &08000 10 &8000 ViewSheet B1.01 &0BFE6 10 &BFE6 spare &0C000 11 &8000 EDIT 1.50 &0FED8 11 &BED8 spare &10000 12 &8000 BASIC 4.32 &14000 13 &8000 ADFS 2.03 &17E5F 13 &BE5F spare &17F8E 13 &BF8E BASIC relocation table &18000 14 &8000 VIEW B3.30 &1C000 15 &8000 TERMINAL 1.20 + TUBE, SRAM & MOS code &1F900 15 &B900 Character setAll code between locations &4000 and &06EFF and between &08000 and &1C000 in the system ROM can be completely replaced. DFS and ADFS share code and must be in ROM slots four places apart so they can be switched between with EOR #4.