Specifications
Chapter 4: Theory of Operation
31
Memory
The following memory options are supported: 8 Mbytes, 16 Mbytes, and 32 Mbytes. One
SIMM socket is available. For memory upgrade instructions, see Appendix D, Memory.
Memory Map
The 2
32
byte physical address space seen by the Intel486 occupies three areas:
• Addresses between 0 and 1 Mbytes, which are largely defined by the IBM PC/AT
architecture.
• Addresses between 1 Mbytes and 256 Mbytes, which largely depend on how much
DRAM is installed in the EPC-8A.
• Addresses above 256 Mbytes, which provide direct mapping to the VMEbus with a
variety of address modifiers and byte orderings. For more information about this
feature, see Chapter 5, Programming the VMEbus Interface.
* 0C8000–0DFFFF may be used either as page frames (i.e. for Ethernet, etc.) or may be
used by DOS as upper memory blocks if an EMM driver E is installed or may be used for
BIOS extensions.
BIOS ROM and ROM Shadowing
The EPC-8A contains a Flash boot device (chip) as its BIOS ROM. The BIOS ROM is
mapped into the top of the processor’s 32-bit address space. The BIOS consists of an 16K
boot block and a System BIOS combined with a VGA BIOS in a 128 Kbyte partition. The
Flash boot device is memory addressed and resides in the last 512 Kbytes of system
memory at address FFF80000H to FFFFFFFFH. The layout is described in Figure 4-2.
Table 4-1. Memory at addresses between 0 and 1MB (0FFFFFh)
Range Content
000000 – 09FFFF DRAM (first 640 KB)
0A0000
– 0BFFFF
Almost always used by a video controller as video RAM.
If the onboard VGA is populated and enabled, then this memory is
accessed over the CPU local bus. Otherwise, it is mapped to the
EXM interface.
0C0000 – 0C7FFF
Write
-protected DRAM containing video BIOS
0C8000 – 0DFFFF* Uncommitted; mapped to EXM interface.
0E0000 – 0EFFFF Either is mapped to EXM interface or is window into the VMEbus.
Mapping controlled by bit-1 of register 8102h.
0F0000 – 0FFFFF Write-protected DRAM containing system BIOS
Table 4-2. Extended memory address space for a 4MB EPC-8A
Extended memory Range Content
8MB EPC-8A
00100000 007FFFFF 7 Mbytes DRAM extended memory
00800000 00FFFFFF Uncommitted; mapped to EXM interface
16MB EPC-8A
00100000 00FFFFFF 15 Mbytes DRAM extended memory
32MB EPC-8A
00100000 01FFFFFF 31 Mbytes DRAM extended memory