Zenith Z-100 Series, Reference Manual

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Page 4.10
Central Processing Unit
Interrupt 12H — Determine Memory Size
When this function is called, a value representing the number of contiguous IK
blocks of user memory is returned in register AX. For example, a value of 256
would indicate that the system contains 256 kilobytes of RAM.
Interrupt IAH&et/Read Time-of-Day
The time set/read function, which permits reading and writing the software time
variables, is
actu ally i n terrupt driven by the CPU's 8253 timer/counter.
If the value in AH is 0, the current time-of-day is read and returned in CX and
DX. CX contains the high part of the count, and DX the low. If the timer has
overflowed to the next day, the contents of AL will NOT be 0; otherwise it will
be 0.
To set the time, the desired high count is placed in the CX register, and the
low count in DX. The contents of AH must NOT be 0 when the interrupt is
called.
The counts mentioned are multiples of the 18.2159 Hz system clock input
(I second = 18.2159). Therefore, I hour would be represented by a count of
approximately 65,577 (18.2159 x 3600); CX would contain the value I, and DX
the value of 41 when the time was read.
Jump Vectors
There are three jump vectors which may be of interest to those creating applications
programs. They are:
F000:FFFO — Power-up reset vector.
F000:FFED — Jump unconditionally to the MFM-150 monitor/debugger.
F000:FFEA — Set machine mode if AL
= OFFH, Z-100 mode will be en-
tered using the Z-319 Bit-Mapped Video Graphics Card.
If AL
= 0, Z-150 mode will be entered using the Z-309 Video
Card.