The writing of the final program code for the application can begin once the system function and hardware have been defined and can be generated in parallel with the detailed hardware design (PC card layout, power supply, etc.) Often the final hardware definition is not possible, however, until some or all of the coding is complete; the memory requirements.
both for Program Memory and Data Memory, may be unpredictable. Also, it may not be possible to predict. in certain time-critical real-time applications, whether the processor will
Prompt 48
Prompt 48 How to Use Prompt 48
have sufficient throughput. "Benchmark" programs, which are typically only the most critical sequences in a complete applications program, are often written in completely coded fonn for
tile
purpose of more exactly predicting memory and throughput requirements.Throughput is defined loosely as the •• amount of computing" that a system can accomplish in a givell time interval. Although a fast processor like the MCS-48 has throughput
"overkill" for most applications, it is easy to conceive that a sufficiently challenging real-time ~plication would overtax its processing power. For example, in some industrial control application, a feedback loop between •• sensing" and •• correcting" might need to be repeatedlYiestablished very quickly, say within 1/l00th of a second. Such a fmal, dedicated applicatiolilS program may be able, in addition to any general ,. housekeeping" or record-keeping duties, to periodically read the current outside-world data appearing at an input port; to penonn data analysis calculations; to compute a feedback or cOlTCCtion factor; and to write this to an output port - all within perhaps 1/l00th of a second.
If benchmlU"k programs are carefully-selected and completely coded, it is possible to make literal and.accurate calculations for the time required to execute them.
One
simply counts the num~r of bytes in the benchmark program (object code) and multiplies by the instruction cycle time of the MCS-48. Assuming a clock frequency of 3 MHz (3 million cycles per second), the basic instruction cycle for the fetching/executing of a program byte would be ~.O microseconds long. (Reference theMeS -48 Microcomputer User's Manual.) Note that most MCS-48 instructions generate only one byte of object code, but that many have operands requiring a second byte.The whole process of applications software development, from program design to fmal coding, is described in Paragraph 6-6.
6-6. Arogramming Techniques
The first part of this section is aimed primarily at beginning or intermediate assembly language l>rogrammers. While it is not sufficient as a general introduction to assembly language programming, it is intended to present concepts allowing efficient software development in the MCS-48 environment. The advanced programmer may wish to spend some time briefly examining the subsections on Program Design and Program Test and Debugging for interest's sake.
The subsections:
Assembling IMP and CALL Instructions, Program Memory Paging, and
Prompt -48 Considerations,
are of general interest as they discuss aspects pertaining specifically to the MCS-48 family or Prompt 48.
The MCS·48 Assembly Language Programming Manual should be consulted as a detailed reference for all MCS-48 CPU software.
6-7.
PrQlram DesignThe first step in the design of any system, hardware or software, is to defme the problem.
Only when the exact function of the application is determined can the resources necessary to execute that function be determined.
A common phrase in programming these days is '·top-down" program design. By this we mean that the designer divides the problem into smaller separate sections to be solved separately, The words "'top-down" describe the hierarchial or pyramid-like way in which this division is made. As an example,let's say that we are to design a program which will
How to Use Prompt 48
6-4
allow a particular MCS-48 system to function as a stopwatch; perhaps we wiD design it to run on Prompt 48 itself. When we say "stopwatch," the precise instructions needed aren't immediately obvious. The problem must be divided into simpler sub-problems. One possible division might be into subsections called: Display Functions, Timer Control Functions, Data Functions, User Input/Output Functions, and so forth. These subsections of the program are then themselves divided and subdivided until the problem is reduced to a number of vastly simpler problems, such as adding 1 to the contents of a given memory location. The final set of simple problems is then solved one at a time, and called the
program
modules.Figure 6-1 shows a possible partial breakdown of the stopwatch problem. While the figure shows the organizational structure of the program, it does not indicate how the modules communicate with one another.
The communication between modules is the second major phase of program design, called designing the modular interfaces, which are simply the ways in which modules pass control and data back and forth. For example, in the stopwatch the User Control Functions (Commands) module must give control to one of its submodules, whose task is to read the keyboard for user commands. The Read Keyboard For Command submodule would examine keys and return control to the calling module. It would also pass data back to the calling module indicating which key, if any, was pressed. The simpler these modular interfaces are kept the easier it is to assemble all modules into a working program. For this reason the modular breakdown process should attempt to separate the problem into sub-problems which depend as little as possible on each other for data.
Stopwatch
UHr Control functions (Com_nds)
Dlspl.y function.
n ... ar Control Function.
Dela functions
-1
-1
SlDp Stopwatch
Fre_ Dlspl.y at Curnnt nma Fr . . (un'-) DI.pl.y Set Tlmato 0
CIe.r LED Dlaptay Enabla LEO Ratr.sh DlspI.y Min utes Dllplay Seconds Dlspl.y Hundredths
R.nln .... r ... rt TI .... r Running Stop n .... r Running Chactc n .... r Stalus
Set n _ 10 OO:GO.OO Add tID nme
Reid Kayboard lor Command
Figure 6·1. Stopwatch Program Structure
Prompt 48
Prompt 48 How to Use Prompt 48
When a given task must be perfonned the same way in two or more modules, it can-be made into a subroutine. A good example of a subroutine is a multiplication routine. The multiplicat/.on routine receives control, and the two numbers to be multipled, from the calling. routine, multiplies the two numbers together, and returns control and the productto the calling routine. Since subroutines are called from a number of different areas in the program, the address of the caller must be saved in order for control and data to pass backto the calling module. This is accomplished very simply in theMCS-48 Chip-Computers and is described in Paragraph 3-9.
The concept of executive modules is also useful. Briefly, an executive module is any module w~ch controls other modules as subroutines. This idea can be applied at any level in the structure of the program, just as the idea of modules itself. In the stopwatch structure, the User Control Functions module