HP Vectra Technical Reference Manual page 323

• Programmatic Reset. The final reset source is a software initiated hardware reset. A
command is sent to the 8041 controller to pulse the 80286 hardware reset line. Once the
80286 has been placed in the Protected Mode, a hardware reset is the only method available
to return to the Real Mode. POST mayor may not be performed depending upon the shut
down status byte in CMOS.
Once a reset operation has been initiated by one of the four possible sources, the system must
determine if it is a power-on reset. If it is a power-on reset, bit 2 in the 8041 controller's status
port is cleared. POST is performed. A command is sent to the 8041 to set bit 2. If it is not a
power-on reset, bit 2 in the 8041 controller status port is already set. The CMOS shutdown
status byte determines whether POST is performed.
If it is not a power-on reset, the system looks at the shut down status byte (CMOS address
OFH)
to determine whether to perform POST or return from protected mode. If the shut down status
byte is set to one of the values that indicates the system is returning from protected mode, the
reset process will initiate the return from protected mode process. This process is described next.
All other values of the shut down status byte are interpreted as reset commands, and the reset
process will initiate the power-on self test process. The reset process has completed its tasks
when one of these two processes has been invoked.
10.3 Protected Mode Support
The 80286 processor has two modes of operation. Protected mode provides memory protection,
virtual memory addressing, and a 16 MB physical address space. Real mode provides a 1 MB
address space and an 8086 compatible mode. The normal mode of operation of the system is
real mode. However, a few programs use protected mode, for example, VDISC.SYS, the DOS
virtual disc device driver.
The system provides some support to the programmer for use of the protected mode features.
The INT 15H driver provides two functions that support system operation in protected mode.
One of these functions enables data to be moved to and from extended memory. This function
enters protected mode to perform this task, and returns to real mode. The second function
provides a method for programmers to switch into protected mode. These functions are
described in Section 9 of this manual.
314 System Processes