Darmawan Salihun wrote:
What is the BIOS RAM in AMD SB7XX used for?
It will for example be used to store RAM configuration parameters when the system is suspended.
Is it to buffer the BIOS contents from SPI flash chip prior to execution of the very first instruction?
No.
I recall that it's impossible to execute code directly in an SPI chip.
It is not only possible but quickly becoming the most common flash chip interface.
I see. So, there must be some sort of independent microcontroller/microprocessor in the southbridge that "fetches" the contents of the SPI chip and "present" it in a "memory-mapped way" to the CPU.
A microcontroller is too slow. An SPI master is just a simple state machine. SPI is a very simple protocol. Typically you will find that southbridge or worst case superio has an SPI master which subtractively decodes the BIOS memory range onto SPI. Accesses come from the CPU to chipset usually then to LPC and finally to SPI. All these bridges are transparent to the CPU.
Andriy Gapon wrote:
No, I think. "BIOS RAM" is some sort of scratch-pad memory for arbitrary use by software.
Correct.
//Peter