User's Manual

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

Bit Description

1 SMM Space Locked (D_LCK). When D_LCK is set to 1 then D_LCK, GMS, USMM, and the most

significant bit of LSMM become read only. D_LCK can be set to 1 via a normal configuration space

write but can only be cleared by a reset. The combination of D_LCK and LSMM provide convenience

with security. The BIOS can use LSMM=01 to initialize SMM space and then use D_LCK to “lock

down” SMM space in the future so that no application software (or BIOS itself) can violate the

integrity of SMM space, even if the program has knowledge of the LSMM function. This bit also

Locks the DRP and DRP2 registers.

0 E_SMRAM_ERR (E_SMERR). This bit is set when processor accesses the defined memory ranges

in Extended SMRAM (HSEG or TSEG) while not in SMM mode. It is software’s responsibility to clear

this bit. The software must write a 1 to this bit to clear it This bit is Not set for the case of an Explicit

Write Back operation.

Initialization and Usage of “Stolen” Memory

SMRAM Register Bits 7:4 control the theft of memory from Main Memory space for use as Graphics

Local Memory and SMM TSEG memory. The blocks of memory selected by these fields are NOT

accessible as general system RAM. When Bit 5 of the SMRAM register is a “1” the TSEG segment of

memory can ONLY be accessed by the processor in SMM mode (No other agent can access this

memory). Therefore, BIOS should initialize this block of memory BEFORE setting either Bit 5 or Bit 7

of the SMRAM register. The memory for TSEG is “Stolen” first and then the Graphics Local Memory is

“Stolen”. An example of this theft mechanism is:

• TOM equal 64 MB,

• TSEG selected as 512 KB in size,

• Graphics Local Memory selected as 1 MB in size

• General System RAM available in system = 62.5 MB

 General System RAM Range 00000000h to 03E7FFFFh

 TSEG Address Range 03F80000h to 03FFFFFFh

 TSEG “Stolen” from 03F80000h to 03FFFFFFh

 Graphics Local Memory “Stolen” from 03E80000h to 03F7FFFFh