Nascom 2k Tiny Basic
Nascom adapted Dr Wang's Palo Alto Tiny Basic (Dr Dobb's Journal, May, 1976), which was originally written for the 8080. These notes are based on Wang's original explanation of the language, adapted for the Nascom version by DW (2008).
THE LANGUAGE
Numbers
All numbers are integers and must be less than 32767.
Variables
There are 26 variables denoted by letters A through Z. There is also a single array @(I). The dimension of this array is set automatically to make use of all the memory space that is left unused by the program (i.e., 0 through SIZE/2, see SIZE function below). In this Nascom version, the start of the @array in memory is immediately after the end of the current program area).
Functions
There are 3 functions:
ABS(X) gives the absolute value of X.
RND(X) gives a random number between 1 and X (inclusive).
SIZE gives the number of bytes left unused by the program.
Arithmetic and Compare Operators
/ divide.
* multiply.
- subtract.
+ add.
> greater than (compare).
< less than (compare).
= equal to (compare).
£ not equal to (compare).
>= greater than or equal to (compare).
<= less than or equal to (compare).
+, -, *, and / operations result in a value between -32767 and 32767. (-32768 is also allowed in some cases.) All compare operators result in a 1 if true and a 0 if not true.
Expressions
Expressions are formed with numbers, variables, and functions with arithmetic and compare operators between them. + and - signs can also be used at the beginning of an expression. The value of an expression is evaluated from left to right, except that * and / are always done first, and then + and - , and then compare operators. Parentheses can also be used to alter the order of evaluation. Note that compare operators can be used in any expression. For example:
10 LET A=(X>Y)*123+(X=Y)*456+(X>Y)*789
20 IF (U=1)*(V<2)+(U>V)*(U<99)*(V>3) PRINT "YES"
30 LET R=RND(100), A=(R>3)+(R>15)+(R>56)+(R>98)
In statement 10, A will be set to 123 if X>Y, to 456 if X=Y, and to 789 if X<Y. In statement 20, the "*" operator acts like a logical AND, and the "+" operator acts like a logical OR. In statement 30, A will be a random number between 0 and 4 with a prescribed probability distribution of: 3% of being 0, 15-3=12% of being 1, 56-15=41% of being 2, 98-56=42% of being 3, and 100-98=2% of being 4.
Direct Commands
All the commands described later can be used as direct commands except the following three, they can only be used as direct command and not as part of a statement:
RUN
will start to execute the program starting at the lowest statement number.
LIST
will print out all the statements in numerical order.
LIST 120
will print out all the statements in numerical order starting at statement 120.
NEW
will delete all statements.
Abbreviation and blanks
You may use blanks freely, except that numbers, command key words, and function names cannot have embedded blanks.
You may truncate all command keywords and function names and follow them by a period. "P.", "PR.", "PRI.", and "PRIN." all stand for "PRINT". Also the word LET in LET command can be omitted. The "shortest" abbreviation for all keywords are as follows:
A.=ABS F.=FOR GOS.=GOSUB G.=GOTO IF=IF IN.=INPUT L.=LIST N.=NEW N.=NEXT P.=PRINT REM=REMARK R.=RETURN R.=RND R.=RUN S.=SIZE S.=STEP S.=STOP TO=TO
Implied = LET
Statements
A statement consists of a statement number of between 1 and 32767 followed by one or more commands. Commands in the same statement are separated by a semi-colon ";". "GOTO", "STOP", and "RETURN" commands must be the last command in any given statement.
Commands
Tiny Basic commands are listed below with examples. Remember that commands can be concatenated with semi-colons. In order to store the statement, you must also have a statement number in front of the commands. The statement number and the concatenation are not shown in the examples.
REM or REMARK command
REM anything goes
This line will be ignored by TBI.
LET command
LET A=234-5*6, A=A/2, X=A-100, @(X+9)=A-1
will set the variable A to the value of the expression 234-5*6 (i.e., 204), set the variable A (again) to the value of the expression A/2 (i.e., 102), set the variable X to the value of the expression A-100 (i.e., 2), and then set the variable @(11) to 101 (where 11 is the value of the expression X+9 and 101 is the value of the expression A-1).
LET U=A£B, V=(A>B)*X+(A<B)*Y
will set the variable U to either 1 or 0 depending on whether A is not equal to or is equal to B; and set the variable V to either X, Y or 0 depending on whether A is greater than, less than, or equal to B.
PRINT Command
will cause a carriage-return (CR) and a line-feed (LF) on the output device.
PRINT A*3+1, "ABC 123 !@£", ' CBA '
will print the value of the expression A*3+1 (i.e., 307), the string of characters "ABC 123 !@£", and the string " CBA ", and then a CR-LF. Note that either single or double quotes can be used to quote strings, but pairs must be matched.
PRINT A*3+1, "ABC 123 !@£", ' CBA ',
will produce the same output as before, except that there is no CR-LF after the last item is printed. This enables the program to continue printing on the same line with another "PRINT".
PRINT A, B, £3, C, D, E, £10, F, G
will print the values of A and B in 8 spaces, the values of C, D, and E in 3 spaces, and the values of F and G in 10 spaces. If there are not enough spaces specified for a given value to be printed, the value will be printed with enough spaces anyway.
PRINT $X
will XOR 40H with the ascii value of X, allowing printing of some graphic characters etc.
INPUT Command
INPUT A, B
When this command is executed, Tiny Basic will print "A:" and wait to read in an expression from the input device. The variable A will be set to the value of this expression. Then "B:" is printed and variable B is set to the value of the next expression read from the input device. Note that not only numbers, but also expressions can be read as input.
INPUT 'WHAT IS THE WEIGHT'A, "AND SIZE"B
This is the same as the command above, except the prompt "A:" is replaced by "WHAT IS THE WEIGHT:" and the prompt "B:" is replaced by "AND SIZE:". Again, both single and double quotes can be used as long as they are matched.
The $ has the save effect as in "PRINT".
IF Command
IF A<B LET X=3; PRINT 'THIS STRING'
will test the value of the expression A<B. If it is not zero (i.e., if it is true), the commands in the rest of this statement will be executed. If the value of the expression is zero (i.e., if it is not true), the rest of this statement will be skipped over and execution continues at next statement. Note that the word "THEN" is not used.
GOTO Command
GOTO 120
will cause the execution to jump to statement 120. Note that "GOTO" command cannot be followed by a semi-colon and other commands. It must be ended with a CR.
GOTO A*10+B
will cause the execution to jump to a different statement number as computed from the value of the expression.
GOSUB and RETURN commands
GOSUB 120
will cause the execution to jump to statement 120.
GOSUB A*10+B
will cause the execution to jump to different statements as computed from the value of the expression A*10+B.
RETURN
A RETURN command must be the last command in a statement and followed by a CR. When a RETURN command is encountered, it will cause the execution to jump back to the command following the most recent GOSUB command.
GOSUB can be nested. The depth of nesting is limited only by the stack space.
FOR and NEXT Commands
FOR X=A+1 TO 3*B STEP C-1
The variable X is set to the value of the expression A+1. The values of the expressions (not the expressions themselves) 3*B and C-1 are remembered. The name of the variable X, the statement number and the position of this command within the statement are also remembered. Execution then continues the normal way until a NEXT command is encountered.
The STEP can be positive, negative or even zero. The word STEP and the expression following it can be omitted if the desired STEP is +1.
NEXT X
The name of the variable (X) is checked with that of the most recent FOR command. If they do not agree, that FOR is terminated and the next recent FOR is checked, etc. When a match is found, this variable will be set to its current value plus the value of the STEP expression saved by the FOR command. The updated value is then compared with the value of the TO expression also saved by the FOR command. If this is within the limit, execution will jump back to the command following the FOR command. If this is outside the limit, execution continues following the NEXT command itself.
FOR can be nested. The depth of nesting is limited only by the stack space. If a new FOR command with the same control variable as that of an old FOR command is encountered, the old FOR will be terminated automatically.
STOP Command
STOP
This command stops the execution of the program and returns control to direct commands from the input device. It can appear many times in a program but must be the last command in any given statement. i.e., it cannot be followed by a semi-colon and other commands.
EX
will return you to the Nascom operating system (Nasbug T2 or T4).
CW
will write the Tiny Basic program to tape using Nasbug cassette format.
CR
will read the Tiny Basic program from tape using Nasbug cassette format.
MC
will simply jump execution to a machine language program at 0C50H in the Nascom(48 bytes are available here for machine code). Additional notes by DW:
If you want to return from the machine code to the next instruction in your Tiny
Basic program, the first line of machine code at 0C50H must be PUSH DE. Then to
return to Tiny Basic where the Tiny Basic program left off, end the machine code
with POP DE, INC DE, JP F911H. For this, the command "MC" can only be the sole
command or last command on a statement line of Tiny Basic. The stack can be used
for temporary storage, but previous stack contents and the stack pointer must be
preserved on exit from the machine code. To transfer variables A to Z between
machine code and Tiny Basic, the values of these variables in Tiny Basic are
stored as hex values, two bytes each, starting at 0C83H for A, 0C85H for B etc,
up to 0CB5H for Z.
For example, at 0C50H:
0C50 D5 PUSH DE 0C51 21 83 0C LD HL,0C83H 0C54 34 INC (HL) 0C55 D1 POP DE 0C56 13 INC DE 0C57 C3 11 F9 JP 0F911H
And in Tiny Basic:
10 A=32000;MC
20 PRINT A
In this example, the "RUN" command in Tiny Basic will increment the variable "A"
by 1, and so print 32001.
Stopping the Execution
the execution of program or listing of program can be stopped by shift/backspace. Press once to pause (return then continues), or press again to restart TB.
Error Report
there are only three error conditions in TINY BASIC. The statement with the error is printed out with a question mark inserted at the point where the error is detected.
(1) WHAT? means it does not understand you. Example:
WHAT?
210 P?TINT "THIS" where PRINT is mistyped
WHAT?
260 LET A=B+3, C=(3+4?, X=4
(2) HOW? means it understands you but does not know how to do it.
HOW?
310 LET A=B*C?+2 where B*C is larger than 32767
HOW?
380 GOTO 412? where 412 does not exist
(3) SORRY? means it understands you and knows how to do it but there is not enough memory to do it.
Error Corrections
If you notice an error in typing before you hit the CR, you can delete the last character with the backspace key or delete the entire line with shift/backspace.
To correct a statement, you can retype the statement number and the correct commands. Tiny Basic will replace the old statement with the new one.
To delete a statement, type the statement number and a CR only.
Verify the corrections by "LIST nnnn". Pressing shift/backspace will halt listing.
Initialisation
The short initialisation program at 0D50H can be altered according to available
ram.
(ARG1) : start of memory for text (set TXTBGN and TXTUNF accordingly)
STKLMT : lower limit of stack
BSTACK : upper limit of stack
(ARG2) : upper limit of ram
(ARG3) : number of lines on screen for LIST