Integrated Functions (Continued)
4.2 INTERNAL BUS INTERFACE UNIT
The Geode GXm processor’s internal bus interface unit
provides control and interface functions to the internal C-
Bus (processor core, FPU, graphics pipeline, and L1
cache) and X-Bus (PCI controller, display controller, mem-
ory controller, and graphics accelerator) paths, provides
control for several sections of memory, and plays an
important part in the virtual VGA function.
4.2.2 A20M Support
The GXm processor provides an A20M bit in the
BC_XMAP_1 Register (GX_BASE+ 8004h[21]) to replace
the A20M# pin on the 486 microprocessor. When the
A20M bit is set high, all non-SMI accesses will have
address bit 20 forced to zero. External hardware must do
an SMI trap on I/O locations that toggle the A20M# pin.
The SMI software can then change the A20M bit as
desired.
The internal bus interface unit performs, without loss of
compatibility, the functions that previously required the
external pins IGNNE# and A20M#.
This maintains compatibility with software that depends
on wrapping the address at bit 20.
The internal bus interface unit provides configuration con-
trol for up to 20 different regions within system memory. It
provides 19 configurable memory regions in the address
space between 640 KB and 1 MB, with separate control
for read access, write access, cacheability, and PCI
access.
4.2.3 SMI Generation
The internal bus interface unit can generate SMI inter-
rupts whenever an I/O cycle in the VGA address range is
3B0h-3BFh and 3C0h-3CFh. An I/O cycle to 3D0h-3DFh
can be trapped. In case an external VGA card is present,
the Internal Bus Interface Unit default values will not gen-
erate an interrupt on VGA accesses. (Refer to Section
5.2.3.1 “SMI Generation” on page 168 for instructions on
how to configure the registers to generate the SMI inter-
rupt.)
The memory configuration control includes a top-of-mem-
ory register and hardware support for VGA emulation
plus, the capability to program 20 regions of the memory
map for different ROM configurations, and to locate mem-
ory-mapped I/O.
4.2.1 FPU Error Support
4.2.4 640 KB to 1 MB Region
The FERR# (floating point error) and IGNNE# (ignore
numeric error) pins of the 486 microprocessor have been
replaced with an IRQ13 (interrupt request 13) pin. In DOS
systems, FPU errors are reported by the external vector
13. This mode of operation is specified by clearing the NE
bit (bit 5) in the CR0 register. If the NE bit is active, the
IRQ13 output of the GXm processor is always driven inac-
tive. If the NE bit is cleared, the GXm processor drives
IRQ13 active when the ES bit (bit 7) in the FPU Status
Register is set high. Software must respond to this inter-
rupt with an OUT instruction of an 8-bit operand to F0h or
F1h. When the OUT cycle occurs, the IRQ13 pin is driven
inactive and the FPU starts ignoring numeric errors. When
the ES bit is cleared, the FPU resumes monitoring
numeric errors.
There are 19 configurable memory regions located
between 640 KB and 1 MB. Three of the regions are
A0000h-AFFFFh,
B0000h-B7FFFh,
and
B8000h-
BFFFFh. The area between C0000h and FFFFFh is
divided into 16 KB segments to form the remaining 16
regions. Each of these regions has four control bits to
allow any combination of read-access, write-access,
cache, and PCI-access capabilities (Table 4-11 on page
102).
In addition, each of the three regions defined in the
A0000h-BFFFh area of memory has a VGA control bit that
can cause the graphics pipeline to handle accesses to
that section of memory (see Table 5-3 on page 170).
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