J131 SCPC88 Processor Board Manual

5. Memory Map

The EPROM sizes are set by LK6-9, and can be from 16K (27128) to 64K (27512). The 256K DRAM is selected by the signal /CAS. Wait-states are not produced for on-board memory. DRAMs with access times shorter than 210 ns, and ROMs faster than 250 ns will run up to 7.16 MHz).

If a memory access is made to an address which is within neither on-board RAM or ROM, the STEbus will be selected automatically. This may result in a bus timeout if no actual memory exists on the bus.

Memory map

00000
3FFFF
256K onboard DRAM (fitted as standard)
40000 PC RAM expansion area (up to 640K maximum) STEbus accesses
A0000
BFFF
EGA Screen RAM
C0000
C3FFF
SPEGA BIOS ROM
D0000
DFFFF
 
E0000
EFFFF
SCPC88 onboard expansion EPROM
F0000
FFFFF
SCPC88 onboard BIOS EPROM (64K reserved area)