Intel 80386

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PHYSICAL DESIGN AND DEBUGGING
11.5.5 Other Simple Diagnostic Software
Other simple programs can be used to check the other operations the system must perform.
The program described here is longer than the 4-byte program illustrated previously; it tests
the abilities to write data into RAM and read the data back to the 80386.
This second diagnostic program, shown in Figure 11-12, is also suitable for placing into
EPROMS. Because this diagnostic loops back to itself, the ADS# LED should glow contin-
uously, just as it does when running the 4-byte program.
The program in Figure 11-12 is based on the assumption that hardware exists to report
whether the data being read back from RAM is correct. This hardware consists of a writable
output latch that can display a byte value written to it. The byte value written is a function
of the RAM data comparison test. If the data is. correct, the byte value written is AAH
(10101010); if the data is incorrect, 55H (01010101) is written.
This diagnostic program is not comprehensive, but it does exercise EPROM, RAM, and an
output latch to verify that the basic hardware works.
The program is short (45 bytes) to be easily understood. Because it is short and because it
loops continuously, a logic analyzer or even an oscilloscope can be used to observe system
activity.
This program can be written in ASM86 assembly language. Because the primary purpose of
this program is to exercise the system hardware quickly, the 80386 is not tested extensively,
and Protected Mode is not enabled.
The diagnostic software verifies the ability of the system to perform bus cycles. The 80386
fetches code from the EPROMs, implying that EPROM read cycles function correctly.
Instructions in the program explicitly generate bus cycles to write and read RAM. The data
value read back from RAM is checked for correctness, then a byte (AAH if the data is
correct, 55H if it is not) is output to the 8-bit output latch. The program then loops back to
its beginning and starts over.
After the source code is assembled, the resulting object code should be as shown in
Figure 11-l3.
11.5.6 Debugging Hints
The debugging approach described in this section is incremental; it lets the programmer
debug the system piece by piece. If even the simple 4-byte program does not run, a logic
analyzer can be used to determine where the problem is. At the very least, the 80386 should
be initiating a code fetch cycle to EPROM.
11-14