GXM datasheet(169/244 Pages) NSC

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Virtual Subsystem Architecture (Continued)

control bit determines whether or not an SMI interrupt is

generated for the corresponding region. The purpose of

this hardware is to allow the VGA emulation software to

disable SMI interrupts in VGA memory regions that are

not currently displayed.

For direct display modes (8 BPP or 16 BPP) in the display

controller, Virtual VGA can operate without SMI genera-

tion.

The SMI generation circuit on the GXm processor has

configuration registers to control and mask SMI interrupts

in the VGA memory space.

5.2.3.2

VGA Memory Addresses

SMI generation can be configured to trap VGA memory

accesses in one of the following ranges:

A0000h to AFFFFh (EGA,VGA),

B0000h to B7FFFh (MDA),

or B8000h to BFFFFh (CGA).

Range selection is accomplished through programmable

bits in the VGACTL register (Index B9h). Fine control can

be exercised within the range selected to allow off-screen

accesses to occur without generating SMIs.

SMI generation can also separately control the following

I/O ranges: 3B0h to 3BFh, 3C0h to 3CFh, and 3D0h to

3DFh. The BC_XMAP_1 register (GX_BASE+8004h) in

the Internal Bus Interface Unit has an enable/disable bit

for each of the address ranges above.

5.2.3.3

VGA Configuration Registers

Table 5-2 summarizes the VGA Configuration Registers.

Detailed register/bit formats are given in Table 5-3.

5.2.3.4

VGA Control Register

The VGA control register (VGACTL) provides control for

SMI generation through an enable bit for memory address

ranges A0000h to BFFFFh. Each bit controls whether or

not SMI is generated for accesses to the corresponding

address range. The default value of this register is zero so

that VGA accesses will not be trapped on systems with an

external VGA card.

5.2.3.5

VGA Mask Registers

The VGA Mask register (VGAM) has 32 bits that can

selectively mask 2 KB regions within the VGA memory

region A0000h to AFFFFh. If none of the three regions is

enabled in VGACTL, then the contents of VGAM are

ignored. VGAM canbeusedtoprevent theoccurrenceof

SMI when non-displayed VGA memory is accessed. This

is an enhancement that improves performance for double-

buffered applications only.

Table 5-2. VGA Configuration Registers Summary

Index

Name

Description

Default

B9h

VGACTL

VGA Control Register

00h (SMI generation disabled)

BAh-BDh

VGAM

VGA Mask Register

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