Integrated Functions (Continued)
4.2 INTERNAL BUS INTERFACE UNIT
The GXLV processor’s internal bus interface unit provides
control and interface functions to the C-Bus and X-Bus.
The functions on C-Bus include: processor core, FPU,
graphics pipeline, and L1 cache. The functions on X-Bus
include: PCI controller, display controller, memory control-
ler, and graphics accelerator. It provides attribute control
for several sections of memory, and plays an important
part in the Virtual VGA function.
4.2.2 A20M Support
The GXLV 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 functions which
previously required the external pins IGNNE# and A20M#.
This will maintain 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.
This includes a top-of-memory register and 19 config-
urable memory regions in the address space between 640
KB and 1 MB. Each region has separate control for read
access, write access, cacheability, and external PCI mas-
ter access.
4.2.3 SMI Generation
The Internal Bus Interface Unit can generate SMI inter-
rupts whenever an I/O cycle is in the VGA address ranges
of 3B0h to 3BFh, 3C0h to 3CFh and/or 3D0h to 3DFh. If
an external VGA card is present, the Internal Bus Inter-
face reset values will not generate an interrupt on VGA
accesses. (Refer to Section 4.6.3 “VGA Configuration
Registers” on page 162 for instructions on how to config-
ure the registers to enable the SMI interrupt.)
In support of VGA emulation, three of the memory regions
are configurable for use by the graphics pipeline and three
I/O ranges can be programmed to generate SMIs.
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. Emulation of 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 GXLV processor is
always driven inactive. If the NE bit is cleared, the GXLV
processor drives IRQ13 active when the ES bit (bit 7) in
the FPU Status Register is set high. Software must
respond to this interrupt with an OUT instruction contain-
ing 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, A0000h
to AFFFFh, B0000h to B7FFFh, and B8000h to BFFFFh,
are typically used by the graphics subsystem in VGA emu-
lation mode. Each of the these regions has a VGA control
bit that can cause the graphics pipeline to handle
accesses to that section of memory (see Table 4-37 on
page 163). The area between C0000h and FFFFFh is
divided into 16 KB segments to form the remaining 16
regions. All 19 regions have four control bits to allow any
combination of read-access, write-access, cache, and
external PCI Bus Master access capabilities (see Table 4-
10 on page 106).
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