Testing for sideways RAM
The standard way to test if a sideways ROM/RAM bank contains RAM is to try to change the binary version number at &8008. If you can change it, it must be RAM, if you can't change it, it must be ROM (or empty). You use the binary version number as that is defined as only ever displayed and not affecting the operation of any code. For example, if you changed &8000 and the test code failed to restore it (eg, Break was pressed) then you would be left with non-functioning code at &8000.
Testing for sideways RAM
\ Code to test for sideways RAM \ ----------------------------- \ This code must run from main memory \ The bank to be tested must be paged in \ Returns Y=0,1 for ROM,RAM LDY #0 :\ Preload Y=0 for ROM LDA &8008:EOR #&AA:STA &8008 :\ Modify version byte CMP &AAAA:CMP &8008:BNE RomTestDone :\ NE=ROM or empty EOR #&AA:STA &8008 :\ Restore byte INY :\ Y=1 for RAM .RomTestDone
The test is done twice as with some systems the data bus floats so that an empty ROM socket looks like it is RAM. Testing twice and testing a different location allows the data bus to settle so an empty socket is seen as empty.
This routine finishes with Y holding 0,1 for ROM,RAM, giving zero for non-writable memory and non-zero for writable memory.
Testing for EEPROM
It is possible to use EEPROMs (Electrically Erasable Programmable ROMs) with BBCs/Masters/Electrons,    but when testing when EEPROM is present you have to be careful as whenever a write is made to the EEPROM it effectively vanishes for 20,000 instruction cycles while it processes the write.  So, you must make sure that the EEPROM isn't inadvertantly called during this write process and must wait until the write process finishes and the EEPROM contents are visible again.
\ Code to test for sideways RAM or EEPROM \ --------------------------------------- \ This code must run from main memory \ The bank to be tested must be paged in \ Returns Y=0,1,2 for ROM,RAM,EEPROM PHP:SEI :\ Disable IRQs, prevent background ROM calls LDY #0 :\ Preload Y=0 for ROM LDA &8008:EOR #&AA:STA &8008 :\ Modify version byte NOP :\ Wait for floating bus to settle CMP &8008:PHP :\ EQ=RAM, NE=ROM/EEPROM/empty EOR #&AA:STA &8008 :\ Restore byte .RomTestLp LDA &8008:EOR &8008:BEQ RomTestOk :\ Bytes match ASL A:BPL RomTestOk :\ Empty socket, bit 6 not toggling LDY #2:BNE RomTestLp :\ Toggling, must be EEPROM/empty, Y=2 .RomTestOk PLP:BNE RomTestNotRam :\ NE from earlier, ROM or EEPROM or empty LDY #1 :\ EQ from earlier, must be RAM, Y=1 .RomTestNotRam PLP :\ Restore IRQs
This routine finishes with Y holding 0,1,2 for ROM,RAM,EEPROM, giving zero for non-writable memory and non-zero for some form of writable memory. Other values could be used, for example the AP6 support ROM uses 0, 'R' and 'E'.
- EEPROM writing in SW rom bank?
- EEPROM - the Holy Grail of Sideways Rom?
- P.R.E.S. New and Original AP6
- AT28C256 32Kx8 EEPROM data sheet