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Integrated Functions (Continued)

4.1.4.1

Scratchpad Memory

The scratchpad RAM is a dedicated high-speed memory

cache that contains BLT buffers, SMM header, and a

scratchpad area for display drivers. It provides both L1

cache performance and a dedicated resource that cannot

be thrown out by other system activity. The configuration

of the scratchpad is based on graphics resolution and is

described in Table 4-5.

The scratchpad memory is part of the on-chip L1 cache

memory. The memory size is controlled by bits in the GCR

abled, or sized to 2 KB, 3 KB, or 4 KB. The remaining L1

cachesizeis16 KBminus thescratchpadsize, andall of

the scratchpad area is subtracted from a single way.

The scratchpad memory is used by display drivers and

virtualization software. Because this resource must be

tightly controlled to avoid conflicts, application software

and third-party drivers should avoid accesses to the

scratchpad area.

The display driver creates and manages two BLT buffers

from within the scratchpad area. These BLT buffers are

used to transfer source data from system memory into the

frame buffer, or for destination data from system memory

or the frame buffer. The graphics pipeline accesses the

BLT buffers for many common operations, including Bit-

BLT transfers, output primitives, and raster text. Display

drivers also use a small portion of the scratchpad as an

extended register file, since scratchpad read and write

accesses are very fast compared to normal memory oper-

ations.

The virtualization software uses the scratchpad area to

store critical SMM information, including the SMI header

and SMM system state. No SMM code currently resides in

the scratchpad area, although this is an option for future

products.

When the BLT buffer pointer is used (refer to Table 4-8 on

page 99) addresses outside the scratchpad range will

wrap around back into the scratchpad RAM. Table 4-5

shows the allocation of scratchpad memory for the 2 KB

and 3 KB configurations of the scratchpad. The 2 KB con-

figuration uses GX_BASE+0800h to GX_BASE+1000h.

The 3 KB configuration uses GX_BASE+0400h to

GX_BASE+1000h. These configurations are fixed by the

system BIOS during boot and cannot be changed without

rebooting the system.

Table 4-5. Scratchpad Organization

2 KB Configuration

3 KB Configuration

Description

Offset

Size

Offset

Size

GX_BASE + 0EE0h

288 bytes

GX_BASE + 0EE0h

288 bytes

SMM scratchpad

GX_BASE + 0E60h

128 bytes

GX_BASE + 0E60h

128 bytes

Driver scratchpad

GX_BASE + 0B30h

816 bytes

GX_BASE + 0930h

1328 bytes

BLT Buffer 0

GX_BASE + 0800h

816 bytes

GX_BASE + 0400h

1328 bytes

BLT Buffer 1

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