IBM 6400 series Maintenance Manual page 153

Load Boot Code into DRAM — Flash memory cannot be accessed
while flash is being detected, cleared, or programmed. This prevents
boot code from running in flash because the opcodes cannot be fetched.
This is solved by copying the program into DRAM and executing the
program out of DRAM. The boot code is position-independent in order for
the program to operate at a different address. The interrupt vector table
is also move to DRAM and the vectors are changed to reflect the new
address.
Detect Flash — The boot code supports many different kinds of flash
memory, permitting any mixture of flash in the four memory banks. The
only requirement is that flash must exist in bank 0 and have boot code
programmed on it. The boot code displays flash configuration,
manufacturer's part number, size, and start address out the debug serial
port.
Display VX Registers — The boot code displays the values of the VX
registers out of the debug serial port.
Check for Pedestal — Boot code uses the operator panel to determine
if the printer is a pedestal or floor cabinet model. Boot code needs to
know the difference so the exhaust fan fault can be disabled on a
pedestal model. "FAN DISABLED" is printed out at the debug serial port
of a pedestal model.
Check for Download Mode — Boot code checks whether the Cancel
and Start keys were pressed at power up. If so, the printer enters
download mode. If not, the printer continues the boot routines.
Check for Diagnostics Mode — Boot code checks whether the
Eject/Restore and Stop keys were pressed at power up. If so, the
printer enters the Boot Diagnostics menu. If not, the printer continues the
boot routines.
Check for Remote Download Mode — Boot code checks if the user
remotely requested the printer to enter download mode. If so, the printer
enters download mode. If not, the printer continues boot routines.
Check for Flash Optimize — Boot code checks if the user requested
the flash file system to be optimized at power up. If so, flash memory is
optimized at this time. If not, the printer continues boot routines.
Check EC Program — The EC calculates a checksum of the program
stored in its flash memory. If there is problem with its program, the EC
puts the message "nopg" in the ECDC communications register. If the
boot code detects this message, it displays "ERROR: EC PROGRAM
NOT VALID" on the LCD. If the EC processor does not respond within
three seconds, the boot code sends "EC STOPPED AT STATE XXXX" to
the LCD, where XXXX is one of these states:
Troubleshooting 153