Vector 4 - Page 60

VECTOR 4
TECHNICAL INFORMATION
2.3 HOW THE PROM SUBSYSTEM FUNCTIONS
Summary: The PROM Subsystem consists of a standard 2732, port
decoder and other circuitry. This section discusses how these
components function.
A, Circuitry used by PROM Subsystem
The PROM is directly addressed by non-multiplexed lines ZA0-ZA1ll. Its output
lines are connected to a 74LS244 buffer which is enabled by a port decoder
subsystem. This subsystem uses the following signals.
- ROMENBL| Generated by the Subsystem Port Register (<D11>) through the use
Port 02H or 03H.
MEMRD Generated by the CPU System- a combination of the MREQ| and
RD| signals.
Z12-7215 Non-multiplexed address lines.
This PROM Subsystem is enabled during the booting procéss and disabled after
the Extended CP/M Operating System has been loaded into memory. This
procedure is described in the next section. |
B. Booting Process
The PROM contains a 4K system program called the Executive (See EXECUTIVE
USER'S MANUAL). This program contains a small video and keyboard driver,
several initialization routines, a testing procedure and two disk booting
programs. These disk booting programs are disk specific and are used to load
in the system tracks from the 5 1/4 inch floppy and the 5 1/4 inch Hard Disk.
The following sequence of events occurs during the RESET process.
1. The Executive program, stored within the PROM located at U35, is locaded
into the system memory map at 00000H ("Global Address"). See Exhibit
I-8.
09-01-82
7200-0001
I1
2-17