Programming and Program Execution
About programming and executions of
program
In most modes of usage, including use as
part of intelligent instruments, computers are involved in
manipulating data. This requires data values to be input, processed
and output according to a sequence of operations defined by the
computer program.
Programming the microprocessor within an intelligent instrument is not
normally the province of the instrument user; indeed, there is rarely
any provision for the user to create or modify operating programs even
if he/she wished to do so. There are several reasons for this. First,
the signal processing needed within an intelligent instrument is
usually well defined, and therefore it is more efficient for a
manufacturer to produce this rather than to have each individual user
produce near identical programs separately. Secondly, better program
integrity and instrument operation are achieved if a standard program
produced by the instrument manufacturer is used. Finally, use of a
standard program allows it to be burnt into ROM, thereby protecting it
from any failure of the instrument power supply. This also facilitates
software maintenance and updates, by the mechanism of the manufacturer
providing a new ROM which simply plugs into the slot previously
occupied by the old ROM.
Whilst it is not normally therefore a task undertaken by the user of
an intelligent instrument, some appreciation of microprocessor
programming is useful background material. To illustrate the
techniques involved in programming, consider a very simple program
which reads in a value from a transducer, adds a pre-stored value to
it to compensate for a bias in the transducer measurement, and outputs
a corrected reading to a display device.
Let us assume that the addresses of the transducer and output display
device are 0000 and 00C1 respectively, and that the required scaling
value has already been stored in memory address 0100. The instructions
below are formed from the instruction set for a Z80 microprocessor and
make use of CPU registers A and B:
IN A, C0
IN B,100
ADD A, B
OUT C1,A
This list of four instructions constitutes the computer program which
is necessary to execute the required task. The CPU normally executes
the instructions one at a time, starting at the top of the list and
working downwards (though jump and branch instructions change this
order). The first instruction (IN A, C0) reads in a value from the
transducer at address C0 and places the value in CPU register A (often
called the accumulator). The mechanics of the execution of this
instruction consist of the CPU putting the required address C0 on the
address bus and then putting a command on the control bus which causes
the contents of the target address (C0) to be copied on to the data
bus and subsequently transferred into the A register. The next
instruction (IN B,100) reads in a value from address 100 (the
pre-stored biasing value) and stores it in register B. The following
instruction (ADD A, B) adds together the contents of registers A and B
and stores the result in register A. Register A now contains the
measurement read from the transducer but corrected for bias. The final
instruction (OUT C1,A) transfers the contents of register A to the
output device on address Cl.
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