IBM 6400 series Maintenance Manual page 152

Test I/O Clock — The VX ASIC has two clocks for internal timing, a
processor clock and an I/O clock. The processor clock cannot be
checked because the processor will not run without it, but the I/O clock
can be checked. The I/O clock is used for sending data to the operator
panel and to get the time for DRAM refreshes. If the boot code detects a
problem with the I/O clock, the error blink code 4–3–4 is sent to the
diagnostic LED on the CMX board.
Read Operator Panel Keys — The operator panel keys are read to
determine if the user is requesting that the printer be placed in download
mode, diagnostic mode, or debug SIMM disabled. If Cancel and Start
are pressed during start-up, the printer is placed in download mode. If
Eject/Restore and Stop are pressed during start-up, the printer is placed
in diagnostic mode. If any other combination of keys are pressed, the
debug SIMM is disabled. Key states are put in an internal register in the
VX ASIC.
Detect DRAM — The VX ASIC supports up to four banks of DRAM
totalling up to 32 Megabytes. Boot code detects how much DRAM is
located in each bank, and relocates the banks to create one contiguous
block of DRAM. The size and location of each bank is then displayed on
the debug terminal through the debug serial port.
Test DRAM — DRAM is tested by writing a pseudo-random number
sequence to all the DRAM, then reading the pattern back to verify it
against what was originally written. If an error is detected, the message
"ERROR: DRAM AT ADDRESS XXXXXXXX" is displayed on the LCD.
When finished, the DRAM test sets all memory to zero.
Inform the EC Processor — The first 8 Kilobytes of DRAM are reserved
for the EC processor, which must be notified when DRAM is ready to be
used. This is done by putting the message "DCS1" in the ECDC
communications register in the VX ASIC.
Detect Ethernet Adapter — An Ethernet adapter it must be detected
early in the power up sequence so the printer can tell the adapter it has
been detected and continue with its own power up sequence. An
Ethernet adapter takes a long time to complete its internal diagnostic
tests, so the boot code allows the adapter to power up in parallel with the
printer to reduce its effect on overall boot time.
Load Boot Messages — The boot code scans flash memory for OEM
boot messages and loads them into DRAM. The manufacturer's boot
code is the same for all OEMs, and contains a set of default messages.
Boot messages for specific OEMs are stored in the DC code, not the
boot code.
152 Troubleshooting