just to expand on the earlier replies...
The 6502 processor can address 64K of memory. Not a single byte more.
In a BBC B there is main RAM between address zero and address 32767 (32K or 0x7FFF hex). That's half the memory accounted for.
Then between address 32768 (32K or 0x8000 hex) and address 49151 (48K or 0xBFFF) is the sideways ROM area. In an unexpanded machine, this is just the BASIC ROM. More on this in a moment...
Finally between 49152 (48K or 0xC000) and 65535 (64K or 0xFFFF) is the OS ROM area, including the small I/O area.
So the whole memory area of the 6502 processor is completely full!
But Acorn had a solution. It's known as memory bank switching or paging. This is where additional circuitry can switch selected memory into the memory area of the processor. Only one memory bank can be switched (or paged) in at a time. But you can have many of these banks. Acorn called their system sideways ROM. As it was intended originally only for ROM.
However, it is possible for the processor to write to this sideways ROM area. Hence it is possible to put RAM in the sideways ROM area. This is exactly what your Watford Electronics RAM/ROM Board is doing. The two 8K byte RAM chips form one 16K byte sideways RAM bank (or 'slot').
The RAM on the Watford Electronics RAM/ROM Board will not contain any useful data until a utility program (command) copies some useful data to it.
Typically either users load a ROM image from floppy disk (or other mass storage) and copy it to the sideways RAM. When the computer is reset, the OS then sees the ROM image in the sideways RAM and treats it like it would any other sideways ROM.
Some games use available sideways RAM as well.
Mark
The 6502 processor can address 64K of memory. Not a single byte more.
In a BBC B there is main RAM between address zero and address 32767 (32K or 0x7FFF hex). That's half the memory accounted for.
Then between address 32768 (32K or 0x8000 hex) and address 49151 (48K or 0xBFFF) is the sideways ROM area. In an unexpanded machine, this is just the BASIC ROM. More on this in a moment...
Finally between 49152 (48K or 0xC000) and 65535 (64K or 0xFFFF) is the OS ROM area, including the small I/O area.
So the whole memory area of the 6502 processor is completely full!
But Acorn had a solution. It's known as memory bank switching or paging. This is where additional circuitry can switch selected memory into the memory area of the processor. Only one memory bank can be switched (or paged) in at a time. But you can have many of these banks. Acorn called their system sideways ROM. As it was intended originally only for ROM.
However, it is possible for the processor to write to this sideways ROM area. Hence it is possible to put RAM in the sideways ROM area. This is exactly what your Watford Electronics RAM/ROM Board is doing. The two 8K byte RAM chips form one 16K byte sideways RAM bank (or 'slot').
The RAM on the Watford Electronics RAM/ROM Board will not contain any useful data until a utility program (command) copies some useful data to it.
Typically either users load a ROM image from floppy disk (or other mass storage) and copy it to the sideways RAM. When the computer is reset, the OS then sees the ROM image in the sideways RAM and treats it like it would any other sideways ROM.
Some games use available sideways RAM as well.
Mark
Statistics: Posted by 1024MAK — Tue Sep 03, 2024 8:16 pm