Memory and I/O Capability
BATTERY BACKUP
All on-board RAM may be battery-backed by a 3.9 V, 1 Amp-hour
lithium battery installed on the ZT 8809A. The 32 Kbyte RAM drive
and the real-time clock are always battery-backed if the battery is
loaded and jumper W12 is installed (the factory default for ZT 8809A
STD DOS systems). The two RAM sockets (7D1, 9D1) may select
battery backup with jumper W35; the selectable RAM/EPROM
socket (3D1) may select battery backup with jumper W38. The choice
of battery backup of a subset, rather than all of RAM memory, is
designed to conserve battery power and therefore lengthen battery
life.
Note for ZT 8809A STD DOS systems: Battery backup of the
selectable RAM/EPROM socket (3D1) is advantageous when loaded
with RAM because this socket is mapped into the top end of the
system memory (see jumper descriptions for W57-59 on page A-41).
As noted above, a DOS RAM drive may occupy the top end of
system memory and is therefore in this RAM. Thus this RAM drive
becomes a battery-backed RAM disk, capable of storing up to
100 Kbytes in files for a 128 Kbyte RAM.
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