Table 4–38 CPU SubDCB Components
Component
Length
Description
Number of
entries
4 bytes
Number of entries in the SubDCB. Initialized by firmware.
Is 0 if no entries are present.
SubDCB
entries
16 bytes
per entry
An entry describes an MMB found by the firmware.
Initialized by firmware. Maximum number of entries is
four.
Table 4–39 CPU SubDCB Entry Components
Component
Length
Description
SIMM block
16 bytes
MMB SIMM description. This field is an array of eight
elements (SIMM0 to SIMM7). Each element is 2 bytes in
size and contains:
Byte 00 - SIMM size in Mbytes.
Byte 01 - SIMM status. Values for SIMM status (in
hex) are:
A5 = SIMM is good.
B4 = SIMM is broken.
C3 = SIMM is absent.
4.8.3 Page Frame Number Bitmap
The page frame number (PFN) bitmap is a data structure that indicates which
pages in memory are considered usable by the OpenVMS operating system. The
bitmap is built by diagnostics as a side effect of the memory tests run during the
power up sequence.
The bitmap starts on a page boundary and resides at the top of memory. The
bitmap requires 1 Kbyte for each 4 Mbytes of main memory, that is:
•
A 32-Mbyte system requires an 8-Kbyte bitmap
•
A 512-Mbyte system requires a 128-Kbyte bitmap
The bitmap does not map itself or anything above it. There may be memory
above the bitmap which has good and bad pages.
Each bit in the PFN bitmap corresponds to a page in main memory. There is
a one-to-one correspondence between a page frame number (origin 0) and a bit
index in the bitmap. A 1 in the bitmap indicates that the page is good and can
be used. A 0 indicates that the page is bad and should not be used. By default,
a page is flagged bad if a multiple bit error occurs when referencing the page.
Single-bit errors, regardless of frequency, will not cause a page to be flagged bad.
Error Handling and Analysis 4–65