Specifications

Memory and I/O Capability
Also included on-board is a 32 Kbyte static RAM, referred to as the
H: drive in STD DOS systems prior to BIOS Version 3.0 and the R:
drive for BIOS versions of 3.0 and later. This RAM is located in the
address space just below the EPROM on board. The memory address
of this RAM varies with the EPROM sizes selected by jumpers
W57-59.
Write Pr
otection
A special feature is provided to protect the on-board static RAM from
being over-written by errant applications code. This is especially
important for STD DOS systems, where critical system configuration
variables are stored in the on-board 32 Kbyte RAM drive. STD DOS
controls the write protection of this RAM. For non-STD DOS
applications, bit 1 of the register at 037Ah in the printer port controls
a gate for the write signal to the RAM. This bit is not usually needed
by most printers, and is therefore dedicated to this function. Refer to
Chapter 9, "Centronics Printer Port," for details on the printer port bit
definitions. After power-up, the RAM is free to be written to; when a
logical 1 is written to the printer port bit, the RAM becomes write-
protected. Reading this RAM is always possible.
5-3