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GSX-86: Graphics for the
APe
9-44
THE GRAPHICS INPUT/OUTPUT SYSTEM (GIOS)
The GIOS contains the device-dependent code in the GSX-86 system. Its is analo-
gous the CP/M-86 BIOS but pertains to graphics devices only. The GIOS contains a
GIOS file, or device driver, for each of the graphics devices on the system. Each
GIOS file contains code to communicate with a single specific graphics device. A
major difference between the GIOS and the BIOS is that while all device drivers
contained within the BIOS are resident in memory at the same time, only one
graphics device driver is resident at a given time. The active device must be changed
by a request from the application program.
Creating A GIOS File
GSX-86 is distributed with a number of device drivers for popular graphics devices.
The device drivers are listed in Table 9-1. If your devices are included in the table,
you need only to edit the Assignment Table file to ensure that it reflects the logical
device numbering assignments you prefer. However, if your device is not supported,
you must create a driver program for it. You can write a driver in any language, but
at least part of it is usually implemented in assembler due to the low-level hardware
interface required.
Device driver files must be in standard CMD format so they can be loaded by the
GDOS. The driver must provide the functions listed as required in the VDI specifi-
cation and must observe the VDI parameter passing conventions. If the graphics
device itself does not support all the GDOS operations directly, the driver must
emulate the capability in software. For example, if a plotter cannot produce a
dashed line, the driver must emulate it by converting a single dashed line into a series
of short vectors and transmitting them to the plotter.
The CP/M-86 Program Development Aids diskette contains a listing of the device
driver for the APC. Use this as a model if you develop your own device driver.
Device drivers are invoked with a "CALLF" from GSX-86, and should return with a
"RETF". The driver must switch to its own stack for internal use, except for an
allowed overhead for a few pushes to save the caller's context. The following entry
procedure is recommended.
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