Specifications

© 2008 Advanced Micro Devices Inc.
SB600 Early-POST Initialization
AMD SB600 BIOS Developer’s Guide (Public Version) Proprietary
Page 20
3.7 System Restart after Power Fail
The way the system restarts following the power-fail/ power-restore cycle depends both on the
PMIO register 74h [bits 1:0], and the hardware jumper on the SB600 pin ACPWR_Strap.
PMIO
Register 74h
bits[1:0]
Description
00b The system restart will depend on the ACPWR_Strap pin pull up/down state.
Pin = 0 : The system will restart without pressing the power button
Pin = 1 : The system will remain off until the power button in pressed.
01b The system will always restart after the power is restored.
10b The system will remain off until the power button is pressed.
11b At power-up the system will either restart or remain off depending on the state of the
system at power failure. If the system was on when the power failed, the system will
restart at power-up. If the system was off when the power failed, the system will
remain off after the power is restored. Pressing the power button is required to restart
the system.
Notes on programming the PMIO register 74h:
1. PMIO register bits[3:0] should be used for programming. Bits[7:4] are read-only bits and
reflect the same values as bits[3:0].
2. Bit2 is used by the hardware to save the power on/off status. This bit should not be
modified during Software/BIOS programming. The BIOS programmer should always
read the PMIO register 74h, modify bit3 and bits[1:0] as required, and write back the
PMIO register 74h.
3.7.1 Power Fail and Alarm Setup
The state of the machine after the power-fail/power-restore cycle is controlled by PMIO register
74h bits[1:0] as described above. This programming can be over-ridden for the special case when
the alarm is set. When both the alarm and the PMIO register 74h bit3 are set, the system will
restart after the power is restored, regardless of how register 74h bits [1:0] are defined.