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ELITE SERIES USER MANUAL APPEN. C - CP/M NOTES
C.1. Z8O BIOS Call Extensions
The only BIOS call which has had extensions made
to it is the SELDSK vector (BOOT+1BH). Digital
Research defines that, prior to calling, the
[C] register is to be loaded with the drive
number (0-15=A:-P:) which is to be used for any
further disk I/O calls. The remaining registers
should be assumed to be in an undefined state.
Upon return, [HL] must contain the address of
the disk parameter header (DPH) associated with
that drive provided the drive can be selected
(if it exists). If the drive is not selectable,
then [HL] must return containing OOOOH. The
remaining registers (including [C]) can be unde-
fined upon return.
Since CP/M is an 8080 operating system, Digital
Reseach makes no declarations concerning the Z80
[IX] and [IY] registers.
Under the enhanced CP/M the [DE], [IX], and [IY]
registers will return standard values which will
normally be ignored by standard CP/M software.
The [DE] register will return containing the
address of DPBASE, which is the base address of
the disk parameter header table as described in
the Alterations Guide. Due to memory limita-
tions, there are only eight DPWs which limits
CP/M to only drives A:-H:. This is an increase
from the original Microsoft distribution system
which only had space for six DPH’s.
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