
Memory Management
2-11
the ISA bus to exist in the address spaces that are normally decoded by the MMS win-
dows. Thus, if more than 640 Kbyte of system RAM is desired for use by a DOS memory
extender (for example), the MMS page should be programmed to point to system RAM
explicitly. This feature also allows a block of system address space in high memory to be
left open for use by the EMM386 memory manager to map in pages of extended memory
using capabilities provided by 386-class and higher processors.
Another implication of system RAM not being the default device is that if the base
address for MMSA is not set to 0C0000h, only externally decoded devices can utilize the
address space abandoned by MMSA in this example. More specifically, if MMSA is made
to start at 0C8000h, the 32 Kbyte of address space from 0C0000h to 0C7FFFh is usable
only if decoded by an external ISA memory-mapped device (e.g., a VGA ROM).
The address spaces—ROM DOS and ROM BIOS—are implemented as chip-select sig-
nals in the élanSC310 microcontroller to eliminate the need for external address decod-
ing. Cycles to these devices use the ISA command signals, MEMR and MEMW. Thus,
any external device that does not have access to the chip signals generated by the
élanSC310 microcontroller (such as ISA devices) may have address conflicts with cycles
that use the MMS mapping.
There is no separate command-signal option for interfacing to the DOS ROM. It is sug-
gested that the DOS or application ROM be made to physically reside above 1 Mbyte in
the system-address space to avoid conflicts with devices whose decoding is fixed by the
PC/AT architecture.
It is invalid to have ROM BIOS shadowing enabled for a region that contains an enabled
MMS page. If this is done, conflicts will result when accessing the overlapping areas. For
example, if ROM BIOS shadowing is enabled for the address range 0E0000–0EFFFFh
and the MMSA starting address is set to 0C4000h, then MMSA page 7
(0E0000–0E3FFFh) should always remain disabled.
Figure 2-7 on page 2-12 shows an example of MMS mapping between system-address
space and all three address spaces. In this example, there are 8 Mbyte of on-board mem-
ory. One page of ROM-BIOS space, two pages of ROM-DOS space, and two pages of
on-board memory are being mapped. The settings in Table 2-7 on page 2-13 define the
MMS mapping shown in Figure 2-7 on page 2-12.