User's Manual
PMAC Dual-Ported RAM User Manual
Dual-Ported RAM Communications 3
System Memory Map (Most Compatible)
Decimal Hex Function
0 0000
16K 04000
32K 08000
48K 0C000 Standard 640K RAM
64K 10000
⋅ ⋅
For DOS, Programs, etc.
⋅ ⋅
⋅ ⋅
624K 9C000
640K A0000
⋅ ⋅
Usually VGA/EGA Space
688K AC000
704K B0000
⋅ ⋅
Recommended memory space
⋅ ⋅
Monochrome may start at B0000
944K EC000
960K F0000
⋅ ⋅
System BIOS & ROM
1008K FC000
Starting Address
Once you have chosen a used block of memory space, you must now configure the DPRAM to start at that
address. To do this, you must write the segment value (eight highest order bits) of your chosen start
address into locations $786 and $787 of PMAC’s X memory (see I/O Memory Map section of PMAC’s
Users Manual). These are special memory locations in PMAC which tell PMAC where in PC memory to
locate the DPRAM. In addition, you also specify using the 20 or 24-bit DPRAM addressing mode. The
24-bit addressing mode locates the DPRAM above the 1 megabyte range (locations above $FFFFF). (If
using an 8086 or an 8088 microprocessor (IBM PC/XT), only use 20-bit addressing and it is limited to
configuring the DPRAM for locations $FC000 and under). In most cases, no matter which microprocessor
being used (AT, XT, or whatever), locating the DPRAM between $B0000 and $EC000 will be sufficient.
Note:
Most will simply use the Configure|PC Dual-Ported Ram window in the PMAC
Executive program to set the address for DPRAM. The following section explains
how to perform this setup without the Executive program.
The PMAC Executive program Configure|PC Dual-Ported RAM screen specifies
PC memory addresses in terms of memory segments. Each memory segment starts
16 addresses from the pervious segment, so the segment address is like the memory
address without the hex digit (i.e. memory address $D4000 is segment address
$D400).
Bits 0-3 (first four least significant bits) of PMAC location X:$787 as follows:
Bit 0: DPRAM enable/disable (1=enable, 0=disable)
Bit 1: 8K RAM (0=8K)
Bit 2: 24-bit/20 bit addressing enable (1=24 bit, 0=20 bit)