# Acorn BBC Master Compact 128K system ROM In the Acorn BBC Master Compact MOS ROM *IC49* is by default a 64Kbyte ROM/27512 ROM chip, but if putting the jumper *PL12* on the *south* position a 128Kbyte chip can be put there. This is a DIP28 adaptor PCB to place two 27512 EEPROMs instead of a single 27C010. ## The system ROM socket *IC49* Unfortunately the 128Kbyte/1Mbyte chip (27C010) is a DIP32 IC, but the socket on the Master Compact is a DIP28 one. I couldn't find any 1Mbyte parallel EPROM chip that fits on a DIP28 socket, so I guess that at Acorn they probably had in mind to use a mask ROM. It is possible to build a dip 28 to dip 32 adapter to fit a 27C010 EPROM in the *IC49* system ROM socket.  Only 7 ROM space banks (9 to F) are assigned to the *IC49* ROM despite the 27512 EPROM can contain up to 4 16Kbyte ROM images because the MOS ROM isn't showed as bank 10. Only bank switchable ROMs are shown by the **ROM* command. ``` > *ROMS ROM F UTILS 10 <----------- IC49 socket ROM E BASIC 40 <----------- IC49 socket ROM D ACORN ADFS 10 <------ IC49 socket ROM C ? <------------------ IC49 socket with PL12 on south ROM B ? <------------------ IC49 socket with PL12 on south ROM A ? <------------------ IC49 socket with PL12 on south ROM 9 ? <------------------ IC49 socket with PL12 on south ROM 8 ? RAM 7 ? RAM 6 ? RAM 5 ? RAM 4 ? ROM 3 ? ROM 2 ? ROM 1 ? ROM 0 ? ``` ### *PL12* jumper behaviour explanation The MOS ROM is always selected (*CS* pin set *low*) and has the *OE* pin set to *low* if *PL12* is on the *north* position and a 27512 ROM chip is present. Otherwise what should have been a 20C1024 chip(?) will not have an *OE* pin and in place of it there will be the *A16* pin with *PL12* set on the *south* position. ## How to program the system and non-system ROMs for this PCB design The system ROM must contain the `MOS` ROM in its top partition `0x00000000-0x00003FFF` and the non-system ROM must contain `DFS/ADFS`, `BASIC` and `UTILS` ROMs in this exact order in its bottom three paritions respectively in `0x00004000-0x00007FFF`, `0x00008000-0x0000BFFF` and `0x0000C000-0x0000FFFF`. More specifically: \ For the system ROM ``` 0x00000000-0x00003FFF corresponds to no ROM slot and must contain MOS 0x00004000-0x00007FFF corresponds to ROM 9 0x00008000-0x0000BFFF corresponds to ROM A 0x0000C000-0x0000FFFF corresponds to ROM B ``` For the non-system ROM ``` 0x00000000-0x00003FFF corresponds to ROM C 0x00004000-0x00007FFF corresponds to ROM D and must contain DFS/ADFS 0x00008000-0x0000BFFF corresponds to ROM E and must contain BASIC 0x0000C000-0x0000FFFF corresponds to ROM F and must contain UTILS ``` The `*ROMS` command will not list the `MOS` ROM and will list the remaining ones starting from the one at the bottom of the memory from the non-system ROM listed as `ROM F`, through the one at the top of the system ROM listed as `ROM 9`. \ This memory layout is basically the same that we would have had if we were using a `27C010` EPROM because the two `27512` are "emulating" a single `27C010`. \ The system will not boot at all if at least the `MOS` and `UTILS` ROMs aren't palced at the right places. ## ROM ICs | ROM IC socket | EPROM | space bank | size | description | | --- | --- | --- | --- | --- | | IC49 | 27512 or 27C010 | D to F or 9 to F | 64Kbyte or 128Kbyte | system ROM quadruple or octuple image ROM | | IC29 | 27128 | 8 | 16Kbyte | single image ROM | | IC38 | 27128 or 27256 | 1 or 0 to 1 | 16Kbyte or 32Kbyte | edge connector or internal double image ROM | | IC23 | 27128 | 2 | 16Kbyte | single image ROM | | IC17 | 27128 | 3 | 16Kbyte | single image ROM | ## GitHub tool to merge multple ROM images into a single file The following GitHub project can be used to merge 4 ROM images into a single one that can be programmed into a single 27128 EPROM chip [github.com - ROM_images_merger](https://github.com/DanCasterIt/ROM_images_merger) A Windows x86 (32-bit) executable file is pushed into the repository so that the program can be runt without compiling it. Usage example to generate the ROM images for the system and non-system ROMs ready to be programmed: ``` $ ./ROM_image_generator Internet_pc128s_rom/OS51.ROM KERMROM basiced EDITROM Input the memory size in bytes. (0 for 65536): 0 Input the number of equally sized partitions. (0 for 4): 0 Input if to fill blank partitions with ones (1) or zeros (0): 0 Found 4 files. Choose where to place them. partition 0 @ 0x00000000-0x00003FFF filled with file: Internet_pc128s_rom/OS51.ROM partition 1 @ 0x00004000-0x00007FFF filled with file: KERMROM partition 2 @ 0x00008000-0x0000BFFF filled with file: basiced partition 3 @ 0x0000C000-0x0000FFFF filled with file: EDITROM Apply changes? (y or n): y Writing "ROM_image.bin"... $ mv ROM_image.bin ROM_imageA.bin $ ./ROM_image_generator DFSimage Internet_pc128s_rom/ADFS210.ROM Internet_pc128s_rom/BASIC48.ROM Internet_pc128s_rom/UTILS.ROM Input the memory size in bytes. (0 for 65536): 0 Input the number of equally sized partitions. (0 for 4): 0 Input if to fill blank partitions with ones (1) or zeros (0): 0 Found 4 files. Choose where to place them. partition 0 @ 0x00000000-0x00003FFF filled with file: DFSimage partition 1 @ 0x00004000-0x00007FFF filled with file: Internet_pc128s_rom/ADFS210.ROM partition 2 @ 0x00008000-0x0000BFFF filled with file: Internet_pc128s_rom/BASIC48.ROM partition 3 @ 0x0000C000-0x0000FFFF filled with file: Internet_pc128s_rom/UTILS.ROM Apply changes? (y or n): y Writing "ROM_image.bin"... $ mv ROM_image.bin ROM_imageB.bin ``` where `ROM_imageA.bin` goes to the system ROM socket and `ROM_imageB.bin` goes into the non-system ROM one. After programming and installing the *EPROMs, the `*ROMS` command will display the following on the PC ``` > *ROMS ROM F UTILS 10 ROM E BASIC 40 ROM D ACORN ADFS 10 ROM C DFS 92 ROM B Edit 01 ROM A The BASIC Editor FF ROM 9 KERMIT 01 ROM 8 ? RAM 7 ? RAM 6 ? RAM 5 ? RAM 4 ? ROM 3 ? ROM 2 ? ROM 1 ? ROM 0 ? ``` ## Useful links - [BBC Master Compact SERVICE MANUAL](https://www.progettosnaps.net/manuals/pdf/bbcmc.pdf) for the computer schematic and Jumper settings - [Mapped Master Compact Motherboard](http://www.8bs.com/incompmapped.htm) Master Compact motherboard picture with IC descriptions - [Paged ROM](https://beebwiki.mdfs.net/Paged_ROM#Master_Compact) Paged ROM explanation with socket to space bank table and ROMFS header description - [PC128S con una EPROM da 128k ?](https://olivettipc128s.altervista.org/pc128s-con-una-eprom-da-128k/) how to put a 27C010 EPROM into the *IC49* socket - [BBC ROMS](http://www.flaxcottage.com/BBC_ROMS/Default.asp), [MDFS::System.ROMs](https://mdfs.net/System/ROMs/) a collection of ROM software for the Acorn BBC computers - [The BBC and Master Computer Public Domain Library Circuit Diagrams](http://8bs.com/othrdnld/manuals/circuitdiagrams.shtml) high resolution circuit schematics - [BBC Master 128 Cartridge Interface](http://8bs.com/submit/acornappspdf/035.pdf) The BBC Master 128 Cartridge Interface has the same signals as the Master Compact edge connector with no user port pins - [76K 2048x1600 EPROM cartridge](https://mdfs.net/Info/Comp/BBC/Circuits/Master/eprom.gif) Master 128 Cartridge circuit that can be easily adapter to work on the external edge connector of the Master Compact - [Replacing the Master System ROM](https://mdfs.net/Info/Comp/BBC/SROMs/MegaROM.htm) How to build a dip 28 to dip32 adapter to fit a 128Kbyte EPROM ## Images