IBM AT 5170 Technical Reference page 83

Low and High Expansion Memory Bytes (Hex 30 and 31)
Bit 7-Bit 0
Address hex 30-Low-byte expansion size
Bit 7-Bit 0 Address hex 31-High-byte expansion size
Valid Sizes:
0200H
0400H
0600H
3COOH
512K-I/O adapter
1024K-I/0 adapter
1536K-I/0 adapter
through
15360K I/O adapter (15M
maximum).
Note:
This word reflects the total expansion memory above
the 1M address space as determined at power-on time. This
expansion memory size can be determined through system
interrupt 15 (see the BIOS listing). The base memory at
power-on time is determined through the system
memory-size-determine interrupt (hex 12).
Date Century Byte (Hex 32)
Bit 7-Bit 0 BCD value for the century (BIOS interface to
read and set).
Information
Flag
(Hex 33)
Bit 7 When set, this bit indicates that the top 128K of
base memory is installed.
Bit 6 This bit is set to instruct the Setup utility to put
out a first user message after initial setup.
Bit 5-Bit 0 Reserved
Hex addresses 34 through 3F are reserved.
System Board 1-67