Address Decode; Rom - Radio Shack TRS-80 Service Manual

DCLK, the reference clock selected, is output from U127. DCLK
is fed back info U127 tor internal riming reference and is also
fed to the clock inpui of U128 (74LS161). U128 is configured to
preload with a count of g each time it reaches a count of O. This
generates a signal output of TC (128 pin 15) that occurs at the
start of every character time of video output. TC is used to gen-
erate LOADS' (Load Shift Register). QA and QC of U128 are
used to generate SHIFr, XADR7", CRTCLK and LOAD' for
proper timing for the four video modes. QA, QB, and QC which
are referred to as H, I, and J are fed to the Graphics Port J7 for
reference timings of Hires graphics video. Refer to Video Tim-
ing, Figs. 5-3 and 5-4 for timing reference.
5.1.5 Address Decode
The Address Decode section will be divided into two subsec-
tions: Memory Map decoding and Port Map decoding.
5.1.5.1 Memory Map Decoding
Memory Map Decoding is accomplished by a 16L8 PAL (U1 09).
Four memory map modes are available which are compatible
with the Model III and Model 4 microcomputers. A second 16L8
PAL (U110) is used in conjunction with U109 for the memory
map control which also controls page mapping of the 32K RAM
pages. Refer to Memory Maps below.
5.1.5.2 Port Map Decoding
Port Map Decoding is accomplished by three 74LS138s (U87,
U88, and U107). These ICs decode the low order address (AO-
A7) from the CPU and decode the port being selected. The IN'
signal from U108 enables U87 which allows the CPU to read
from a selected port and the our signal, also from U108, en-
ables U88 which allows the CPU to write to the selected port.
U1 07 only decodes the address and the IN" and our signals
are ANDed with the gener~ted signals.
5.1.6 ROM
The Model 4P contains only a 4K x 8 Boot ROM (U70). This
ROM is used only to boot up a Disk Operating System into the
RAM memory. If Model III operation or DOS is required, then
the RAM from location 0000-37FFH must be loaded with an im-
age of the Model III or 4 ROM code and then executed. A sys-
tem program called MODEL A/III is supplied with the Madej 4P
to provide the ROM image for proper Model III operation. On
power-up, the Boot ROM is selected and mapped into location
OOOO-OFFFH. If the Boot ROM is not required after boot up, the
Boot ROM must be mapped out by OUTing to port 9CH with DO
set or by selecting Memory Map modes 2 or 3. In Mode 1 the
RAM is write enabled for the fu II 14K. This allows the RAM area
mapped where Boot ROM is located to be written to while ex-
ecuting out of the Boot ROM. Refer to Memory Maps.
26
The Model 4P Boot ROM contains all the code necessary to
initialize hardware, detect options selecled from the keyboard.
read a sector from a hard disk or floppy, and load a copy of the
Model III ROM-Image (as mentioned) into the lower 14K of
RAM.
The firmware is divided into the following routines:
Hardware Initialization
Keyboard Scanner
Control
Floppy and Hard Disk Driver
Disk Directory Searcher
File Loader
Error Handler and Displayer
RS-232 Boot
Diagnostic Package
Theory of Operation
This section describes the operation of various routines in the
ROM. Normally, the ROM is not addressable by normal use.
However, there are several routines that are available through
fixed calling locations and these may be used by operating sys-
tems that are booting.
On a power-up or RESET condition, the Z80's program counter
is set to address 0 and the boot ROM
is
sWitched-in. The mem-
ory map of the system is set to Mode O. (See Memory Map for
details.) This will cause the Z80 to fetch instructions trom the
boot ROM.
The Initialization section of the Boot ROM now performs these
functions:
1. Disables maskable and non-maskable interrupts
2. Interrupt mode 1 is selected
3. Programs the CRT Controller
4. Initializes the boot ROM control areas in RAM.
5. Sets up a stack pointer
6. Issues a Force Interrupt to the Floppy Disk Controller
to abort any current activity
7. Sets the system clock to 4mhz
8. Sets the screen to 64 x 16
9, Disables reverse video and the alternate character
sets
10. Tests for
< . >
key being pressed'
11. Clears all 2K of video memory
*
This is a special test. If the
<'
:>
is being pressed, then
control is transferred to the diagnostic package in the
ROM. All other keys are scanned via the Keyboard
Scanner.