System information

The READ and WRITE routines should perform several retries (10 is standard) before reporting
the error condition to the BDOS. If the error condition is returned to the BDOS, it reports the
error to the user. The HOME subroutine might or might not actually perform the track 00 seek,
depending upon controller characteristics; the important point is that track 00 has been selected
for the next operation and is often treated in exactly the same manner as SETTRK with a
parameter of 00.
The following table describes the exact responsibilities of each BIOS entry point subroutine.
Table 6-5. BIOS Entry Points
Entry Point Function
BOOT The BOOT entry point gets control from the cold start loader and is
responsible for basic system initialization, including sending a sign-on
message, which can be omitted in the first version. If the IOBYTE
function is implemented, it must be set at this point. The various system
parameters that are set by the WBOOT entry point must be initialized,
and control is transferred to the CCP at 3400 + b for further processing.
Note that register C must be set to zero to select drive A.
WBOOT The WBOOT entry point gets control when a warm start occurs. A warm
start is performed whenever a user program branches to location 0000H, or
when the CPU is reset from the front panel. The CP/M system must be
loaded from the first two tracks of drive A up to, but not including, the
BIOS, or CBIOS, if the user has completed the patch. System parameters
must be initialized as follows:
location 0,1,2 Set to JMP WBOOT for warm starts
(000H: JMP 4A03H + b)
location 3 Set initial value of IOBYTE, if implemented in the
CBIOS
location 4 High nibble = current user no; low nibble current
drive
6.6 BIOS Entry Points CP/M Operating System Manual
6-20