Educational Software Cooperative 4

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ELEMENTARY COMPUTER PROGRAMMING Lesson 4 Copyright 1990 Castle Oaks Computer Services Post Office Box 36082 Indianapolis, IN 46236-0082 In the previous lessons, the necessary information was given for writing a TRAC program, executing a TRAC program, and writing a TRAP program. In this lesson, the information to assemble a TRAP program will be given. First, you must create the TRAP program according to the rules given in Lesson 3. Then you will be ready to assemble the program. The general command line format for using the assembler is: TRAP [source file] [object file] The parameters in brackets are optional. If either one is omitted, the assembler will request the needed file descriptor. A file descriptor may include a drive letter and subcatalogs (if required.) If the object file descriptor is the same as that of an already existing file, the new object file being created will replace the existing one. There are no limitations on the file extensions for either the source file or object file; however, it is recommended that you use a file extension of 'TRA' for source files and 'OBJ' for object files. As you accumulate files, these extensions will help you to identify the types of the files. The assembler is a two pass assembler. On the first pass, all locations are assigned a numeric memory address and those with mnemonic names will be entered into a symbol table for later use. The symbol table can hold only 200 different symbols. Since this is a large number no provision has been included to detect symbol table overflow. Should you have too many symbols, you will probably "lock up" the computer. After the copyright notice appears, the next thing that displays is an "End of Pass 1" message. On the second pass, mnemonic operation codes are assigned numeric values; and mnemonic or relative addresses are assigned their numer- ical equivalents. Each line of assembled code is displayed along with the source code that generated it. During this pass, if any syntactical errors are discovered, an appropriate error message will be displayed, some action will be taken, and the assembly will continue. An error message will appear before the line of code that contains the error. At the end of pass 2, if any errors were de- tected, the number of errors will be displayed. Also during pass 2, the object file will be written. An object file will be created even if the program contains errors. Therefore, if you try to execute a program with syntactical errors the program may abort or something more serious may happen (like "crashing" the computer.) IV-1 When an assembly has been completed successfully, the object program may be executed with TRAC using the methods outlined in Lesson 2. (Note: Even if a program does not have any syntactical errors, it may have logical errors. There is no way that an assembler can detect your logical errors. It is strongly recommended that you manually perform one or more test cases for any programs you create.) In Lesson 3, an example program was shown in its source form (Figure III-8) and the resulting object code (Figure III-8a). Note that the source code is carried over into the object file and when the pro- gram is executed, everything beyond the first 15 columns is treated as a comment. In lesson 1, a flow chart (Figure I-4) and TRAC code (Figure I-5) were given of a method for converting an octal number to decimal. The same program is given here in Figure IV-1 and is included in the software package as FIGIV-1.TRA (see next page). Note that the code generated by the assembler is identical to that that was hand coded for Figure I-5. This resulted because care was taken in re-origin- ing various parts of the program to force this to happen. There was no necessity for doing so except to illustrate that we could force it to happen. You are encouraged to take this source code and make other memory assignments, even rearrange the code to see how the memory assignments change or not. Although you have considerable latitude in making assignments there are some limitations. Note that in the program, where relative addressing was used, the actual number, 2, had to be used whereas elsewhere the index register was referred to by the mnemonic name, I. Therefore, it was necessary that I be made equivalent to 2. Also the definition for the read area was placed last. This was done so that we didn't have to explicitly reserve locations 1002, 1003, and 1004. The re-origining at 0999 could be omitted and the program would function as before but the read area would start at location 1506 instead of 1000. Try this, execute the resulting program and note that it will function the same. Another possible modification would be to move the two lines, 0001 0 I 0 so they are the first two lines of the code and then you could omit the re-origin lines 0009, 1499, and 0999. This would cause the object program to use consecutive locations. STRT would now be location 0003, the actual code would occupy through location 0041, the constants and variables would use 0042 through 47, and the read area would start at location 0048. IV-2 ----------------------------------------------------------------- 0009 0 Origin STRT RD X Read X (Also sets next 4 locations to zero) LD X Load accumulator with X BZSTOP If X is zero jump to halt instruction BN*+4 If X is negative, skip next 3 instructions LD ONE Load +1 ST S Store at S (sign) BU*+6 Branch unconditionally around next 5 LDNEG1 Load -1 ST S Store at S LDZERO Load zero SU X Subtract X (This makes a negative X positive) ST X Store back at X LD ONE Load 1 ST I I=1 using index register 2 LD X Load X SR0008 Shift it right 8 ST Y Store at Y LOOP LD Y Load Y (Note this is the start of a loop) MU ATE Multiply by 8 ST Y Store at Y LD X Load X 2SL0000 Shift left by I (Contents of register 2) SR0008 Shift right 8 (This isolates an octal digit) AD Y Add previous Y ST Y Store back at Y LD I Load I AD ONE Add 1 ST I Store at I SUNINE Subtract 9 to see if done BNLOOP If negative repeat loop LD X Load X MU S Multiply by sign ST X Store at X LD Y Load Y MU S Multiply by sign ST Y Store at Y PC X Print X, Y BUSTRT Branch back to start STOP HT0000 Halt 0001 0 Re-origin to define I I 0 1499 0 Re-origin for constants ZERO 0 Zero ONE 1 One S 0 NEG1 -1 Negative one ATE 8 Eight NINE 9 Nine 0999 0 Re-origin for read area X 0 Y 0 ENSTRT End of code and starting location Figure IV-1 ----------------------------------------------------------------- IV-3 The following flow chart is a "bubble" sort. You enter 10 values (using two reads) then the program makes multiple passes through the data exchanging values where needed until a pass is completed with no exchanges. The sorted values are then printed. | v -------------------------------- / Read A(0),A(1),A(2),A(3),A(4) / -------------------------------- | v -------------------------------- / Read A(5),A(6),A(7),A(8),A(9) / -------------------------------- | v +-----+ | I=0 |<---------------+ | F=0 | | | J=1 | | +-----+ | | | v | / \ | / \ | / \ <= | +---><A(I): >-----+ | | \ A(J)/ | | | \ / | | | \ / | | | | > | | | v | | | +-------------+ | | | | T=A(I) | | | +-------+ | | A(I)=A(J) | | | | STOP | | | A(J)=T | | | +-------+ | +-------------+ | | ^ | | | | | | v | | ------------------------- | +-----+ | | / Print / | | F=1 | | | /A(5),A(6),A(7),A(8),A(9)/ | +-----+ | | ------------------------- | | | | ^ | v | | | | +-------+ | | | | | J=J+1 |<----+ | ------------------------- | | I=I+1 | | / Print / | +-------+ | /A(0),A(1),A(2),A(3),A(4)/ | | | ------------------------- | v not = | ^ | < / \ >= / \ = | +------<I:9>--------------><F:0>-----------+ \./ \./ Exercise IV-1 IV-4 Experiment with exercise IV-1. For example, you do not really need to have an explicit J. It is always equal to I+1. You can elimi- nate it in the following manner. When you reserve space for A, you can give the location following A(0) a different name, say A1. Then everyplace that calls for A(J), you can use A1(I). This technique can be used to facilitate the second read and the second print. That is, give the memory location that corresponds to A(5) a name such as A5. Then you can say RD A5 and PC A5 in order to read and print the second set of A's. | v +---------+ | T(0)=3 | | X=5 | | J=1 | +---------+ | +-------+ v | STOP | ---------- +-------+ / Read XF / ^ ---------- | > | / \ v / \ <= +-----+ <X:XF >----------------->| I=0 |<---------+ \ / +-----+ | \./ | | ^ v | | +------------+ | +-------+ | Q=X/T(I) | | | X=X+2 |<------+ | R=X-Q*T(I) | | +-------+ | +------------+ | ^ | | | | | v | ---------- | = / \ | / Print X / +----------<R:0> | ---------- \./ | ^ | Not = | | v | | --------- | +-------+ +--------+ < / \ >= +-------+ | J=J+1 |<-| T(J)=X |<-<X:T(I)*T(I)>---| I=I+1 | +-------+ +--------+ \ / +-------+ --------- Exercise IV-2 In exercise IV-2 you will build and display a table of prime numbers (i.e. numbers which are divisible only by themselves and one.) In the first box of the flow chart, certain quantities are initialized. You may either do this within the executable program or, you may initialize them by entering their initial values when you reserve memory space for them. IV-5 The program requests a final value. This provides an upper limit for the execution. Should you wish to terminate earlier, you may abort the program by using ^C (control-C). The program starts with only the value, 3, in the table (even though 2 is also a prime, it is the only even prime and therefore we will only test the odd numbers.) Starting with a value of 5, each odd number is divided by each value in the table whose square does not exceed the number being tested to see if there is a remainder. If there is no remainder, the number is not a prime and the program advances to the next number to be tested. If there is a remainder for all tests made, the number is prime and the value is added to the table for possible later use and the next odd number is tested, etc. Code this exercise in TRAP, assemble it, and run it. Note that you do not have to actually store the values, Q and R. Try this varia- tion and other variations you might think of to gain a better under- standing of the use of an assembler and machine code. As further exercises, re-code Figures II-5 and II-8 in TRAP. Assem- ble and execute them. We hope you enjoyed learning TRAC and TRAP. IV-6