address decoding is now performed on the translated address which comes
from the memory mapper, instead of the address coming directly from the
cpu.
This means that you can access the full amount of ram at any address
that it is mapped to.
Based on the 74ls610 but with some slight changes.
The memory mapper works by having a 16x12 ram array. The top 4 bits of
the address are used to index into this array, and the resulting word
replaces those top 4 bits. In this way, a 16 bit address is replaced
with a 24 bit address.
As of now there is no way to write 12 bit values though, so currently
we are using 20 bit addresses.
There is a chip select line that allows you to write into the ram array,
and another chip select that allows you to write to the control word.
Currently the control word is just a single bit, the enable bit.
When not enabled, the 4 index bits are passed straight through, and the higher
bits of the address are replaced with 0, a sort of identity map. Once
enabled, it operates as described above.
Since the bottom 12 bits are left unchanged, the page size is 4kb.
There are no protections so far, but might be added later, as well as
the ability to actually use all 12 bits.
This removes the ram from inside the FPGA. All RAM is now located in the
external SDRAM instead.
The ROM is still in the FPGA to allow easier programming.
Turns out there are some issues with holding the chip select for the
SDRAM controller high for too long, so there is a simple 2-state fsm
which ensures that the chip select is only held for 1 clock cycle for
writes and for as long as it takes to read the data from sdram for
reads.
Upon receiving an interrupt, the corresponding bit in the interrupt
status register will be set and an IRQ will be raised for the CPU. The
cpu can then respond to the interrupt and clear the interrupt by writing
back to the interrupt status register.
This also increases the number of registers to 4, one more for the high
pair of displays, and a final one for a mask register which has not been
implemented yet.
