Arithmetic, Logical, and Compare Operations

       +   addition.
       -   subtraction.
       *   multiplication.
       /   integer division  (note that 14/5 = 2).
      MOD  remainder from division (14 MOD 5 = 4).

The logical operators are:

      NOT  bit-wise logical complement (NOT 0 = -1).
      AND  bit-wise logical AND (3 AND 6 = 2).
       OR  bit-wise logical OR (3 or 6 = 7).
      XOR  bit-wise logical Exclusive - OR (3 XOR 6 = 5).

The compare operators are:

        >  greater than.
        <  less than.
        =  equal to.
       <>  not equal to.
       >=  greater than or equal to.
       <=  less than or equal to.

Expressions

Within each precedence level, the value of an expression is computed from left to right. 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) AND (V<2) or (U>V) AND (U<99) 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 30, Y will be a random number between 0 and 4 with a pre- scribed probability distribution of: 3% being 0, 15 - 3 = 12% being 1, 56 - 15 = 41% being 2, 98 - 56 = 42% being 3, and 100 - 98 = 2% being 4.

Direct Commands

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 and set variables A-Z to zero.

Abbreviation and Blanks

You may truncate all commands keywords and function names and follow them by a period. "P.", "PR.", "PRI.", and "PRIN." all stand for "PRINT". Also, the words LET in the LET command and THEN in the IF command can be omitted. A var- iable name by itself will print the value of that variable on the console.

The "shortest" abbreviation of all command keywords are as follows:

      C. = CALL            D. = DECIMAL     E. = END
      F. = FOR             G. = GOTO      GOS. = GOSUB
      H. = HEX             I. = IF         IN. = INPUT
      L. = LET(Optional)  LI. = LIST        N. = NEXT
     NEW = NEW             P. = PRINT     PRO. = PROM
      R. = RETURN         RA. = RAM       RES. = RESET
     RO. = ROM            RU. = RUN        REM = REMARK
      T. = TO              T. = THEN(Optional)
      Implied = LET   Variable name = PRINT variable 

The following abbreviations are the shortest allowed within a variable name or expression:

      A. = ABS         A. = AND         C. = CBYTE
      D. = DBYTE       M. = MOD         N. = NOT
      O. = OR          R. = RBIT       RN. = RND
      X. = XBYTE       X. = XOR

Statements

Commands

REM or REMARK Command

LET Command

LET A = 234 - 5 * 6: A = A/2: X = A - 100

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), and set the variable X to the value of the expression A - 100 (i.e., 2).

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.

Sequential variables may be initialized by separating sequential expressions by commas.

LET A = 1, 2, 3, A, B * C

Will set variable A to 1, B to 2, C to 3, D to 1 (the updated contents of A), and E to 6.

PRINT Command

PRINT

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., 4), the string of characters "ABC 123 !@#", and the string "CBA", and then a CR-LF. Note that 'single' or "double" quote marks may be used to bracket a string.

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".

Each comma in the above examples causes one or more spaces to be printed until the cursor has reached a tabulation column, which occur every eight spaces. All spaces between variables and character strings may be eliminated by using semicolons instead of commas. (There are no spaces inserted auto- matically before or after numbers or strings.)

PRINT , , , "X="; :X = -1: PRINT X; "!"

Will print the five characters "X=-1!" on one line, starting in column 25.

INPUT Command

INPUT A,B

When this command is executed, Tiny BASIC will print ":" and wait to read in a number from the input device. The variable A will be set to this value. A tab and another ":" is printed and variable B is set to the value of the next number read from the input device. A number being entered may be terminated with any non-numeric character, such as a space or <CR>.

INPUT "WHAT IS THE WEIGHT", A, "AND SIZE", B

This is the same as the command above, except the prompt ":" is replaced by "WHAT IS THE WEIGHT:" and the second ":" is replaced by "AND SIZE:".

INPUT A, 'STRING'; "ANOTHER STRING", B;

The strings and semicolons have the same 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 odd (i.e., if it is true), the commands in the rest of this statement will be executed. If the value of the expression is even or zero (i.e., if it is not true), the rest of this statement will be skipped over and execution continues at the next state- ment. The keyword "THEN" may optionally be used after the expression.

GOTO Command

GOTO 120

Will cause the execution to jump to statement 120. Note that the GOTO command cannot be followed by a 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 command is similar to GOTO command except that the current statement number is remembered.

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 10 * A + B.

RETURN

A RETURN command must be the last command in a statement and followed by CR. When a RETURN command is encountered, it will cause the execution to jump back to the command line 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

The variable X is set to the value of the expression A + 1. The value of the expression (not the expression itself) 3 * B is remembered. The name of the variable X and the statement number are also remembered. Execution continues in the normal way until a NEXT command is encountered.

NEXT X

The name of the variable (X) is checked with that of the most recent FOR com- mand. If they do not agree, an error message is generated. When a match is found, this variable will be set to its current value plus 1. The updated value is then compared with the value of the TO expression also saved by the FOR com- mand. 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.

END Command

END

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 colon and other commands. END is optional as the last line of a program.

Stopping the Execution

The execution or listing of the program can be aborted by pressing the Control-C key on the input device. The command in progress or line being listed will first be completed. Output to the console may be momentarily stopped by typing Control-S. Output will resume when Control-Q is typed.

RESET Command

RESET

Will restart the TB51 interpreter from the beginning, as if a hardware reset were performed.

Error Report

There are only three error conditions in TINY BASIC.

1. WHAT? means it does not understand you. Example: 210 PTINT "THIS" where PRINT is mistyped When this line is executed, TB51 will halt and print "WHAT?".
2. HOW? means is understands you but does not know how to do it. Examples: 310 LET A = B * C + 2 where B * C is greater than 32767 410 GOTO 412 where 412 does not exist When these lines are executed, TB51 will halt and print "HOW?".
3. SORRY means it understands you and knows how to do it, but there is not enough memory to do it. Note: TB51 does not retype the erroneous statement or indicate where the error occurred.

Error Corrections

When numbers are being entered during the execution of an input command, the rubout key will be echoed by a "#" and CR, the current entry will be aborted, and a new number may be entered.

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" and hit the Control-C key as soon as the printing of the line begins. (Note that TB51 will not honor the Control-C until it completes the current line.)

BASIC Program Buffer Data Structure

RAM, ROM, PROM

For example, a program to print the first 10 squares:

      10  FOR A = 1 TO 10
      20  PRINT A * A: NEXT A

could be translated for an external EPROM using ASM51 and the following assem- bly language source lines:

     ORG  1080H
      DW  10
      DB  'FOR A = 1 TO 10', 0DH
      DW  20
      DB  'PRINT A * A: NEXT A', 0DH
      DB  0FFH

(Note -- to insert an apostrophe into a text string such as those above, ASM51 requires two adjacent apostrophes in its place.)

A program located in external PROM will be enabled and executed automatically following a reset if the RxD pin is jumpered to ground. As noted earlier, the auto-baud detection routine will be skipped, so if the program in PROM needs to use serial communications, it will have to initialize the appropriate registers.

The above has been a discussion of the BASIC-like aspects of TB51. In addi- tion to these features, the following monitor-like capabilities are provided. Note that these capabilities are fully compatible with, and may be freely inter- changed with, the commands and statements described thus far.

DBYTE, RBIT, CBYTE, XBYTE Variable Arrays

      SFR     DBYTE      SFR     DBYTE
      ---     -----      ---     -----
      P0      128        SCON    152
      P1      144        SBUF    153
      P2      160        IE      168
      P3      176        IP      184
      TCON    136
      TMOD    137
      TL0     138
      TL1     139
      TH0     140
      TH1     141

Attempting to read or write registers other than those above will cause an error message.

PRINT DBYTE 144

Will print the input data present at P1 to the console.

LET DBYTE 144 = DBYTE 8 + 1

Will read the contents of register 0 of register bank 1, increment that value, and output it on P1.

PRINT DBYTE (DBYTE 8)

Will print the contents of the internal RAM location or SFR indirectly addressed by the same register.

Reading a bit address will return a one or zero, depending on the state of that bit. The value written to a bit address is the least significant bit of the expression specified. Other bits of the affected register are not changed.

RBIT 144 = RBIT 145 AND NOT RBIT 146

Will set P1.0 only if P1.1 = 1 and P1.2 = 0.

XBYTE is used to access up to 64K bytes of external data memory controlled by the 8051 RD and WR strobes. CBYTE is used to read internal program memory at addresses less that 4096, and external program memory controlled by the PSEN strobe. Writing to CBYTE addresses uses the WR strobe, the same as the XBYTE command.

Notice that the implicit PRINT and LET commands can be used to examine reg- isters and initialize sequential memory locations using a syntax similar to that of Intel's In-Circuit Emulators.

DBYTE 140

Will print the current contents of the byte TH0.

XBYTE 8192 = 0, 1, 2, 3, 4, 5

Will initialize six bytes of external data RAM with values from 0 to 5.

HEX, DECIMAL Commands

To facilitate monitor interaction, TB51 also has a hexadecimal operating mode invoked by the command HEX. In this mode all variables are treated as unsigned 16-bit values. Numbers may be entered as a combination of decimal and hexadeci- mal digits, provided the first digit is between 0 and 9. If more than four dig- its are entered, the higher order bits are ignored. In the interest of compati- bility with other Intel software conventions, a hex number may be optionally followed by an "H", but its presence or absence will not affect the radix as- sumed. Numbers will be printed out in HEX mode as two to four hex digits fol- lowed by an "H". All arithmetic will be performed modulo 2^16 (65536), and arithmetic overflow (other than division by zero) will be ignored.

Decimal operation may be restored by the command DECIMAL.

Caution must be exercised when changing modes while a program is being developed. Line numbers preceding stored commands are interpreted and saved assuming the radix in effect as the line is entered. Line numbers included in a command (as in GOTO 20) are interpreted using the radix in effect as the statement is executed.

CALL Command

When the machine language program begins executing, register bank 3 will be enabled, but all CPU registers will be undefined. The 8051 stack pointer will hold 57H; internal RAM locations 56H and 57H contain a return address to within the TB51 interpreter. RAM locations 68H - 7FH are available for the user's stack. In addition, register bank 2, register bank 3, and RAM locations 20H - 27H are not touched by TB51 (except in the single-chip mode described below) and may be used by the machine language program. Notice that the DBYTE accessing capability allows these variables to be read, monitored, and written by the interpreter programs.

Single Chip Operation

1. With no program line buffering, BASIC and machine language programs can- not be written and edited. However, single and compound command lines may be executed from the keyboard.
2. Variables used by BASIC commands and expressions are automatically stored in internal RAM, rather than external. To conserve this RAM, only 12 such var- iables (A-L) are supported, overlapping internal RAM locations 10H - 28H. R0 of register bank 1 would be the same location as the high order byte of variable E. Conversion to internal variable memory is automatic, following a hardware reset.

Errata

1. The random number generator seed is initialized to 0 on reset, and is thereafter permuted by the recurrence relation

         R(n) = [(25,173 * R(n-1)) + 13,849] mod 65536.

   (reference "Programming in PASCAL" by Peter Grogono, pg. 117). A characteristic of this algorithm is that the low-order bits of the seed produce a cyclically repeating pattern. If the argument of the RND function is a low-order power of two (such as 4, 8, or 16) then the values returned by the function will follow a similar sequence. One solution is to obtain a random number within wider limits, then eliminate certain cases:

      120  A = RND(3): IF A = 3 GOTO 120
      Will produce a random number between 1 and 2.

2. If the upper limit of a FOR-NEXT loop (the TO clause) is equal to the maximum integer representable in 16 bits (32767 in DECIMAL mode, 0FFFFH in HEX mode), the loop will not terminate. TB51 increments the loop variable before testing it, with the result that the loop variable can overflow, wrapping around to the bottom of the number range (32767 becomes -32768, 0FFFFH becomes 0H). It then passes the comparison test to the upper bound of the loop. For example:

      120  FOR I = 32766 to 32767
      Will loop forever, with I taking all values from -32768 to 32767.

3. The value -32768 is legal when in DECIMAL mode, but cannot be printed or input. The following program will halt with a HOW? message:

      10  A = -32767
      20  A = A - 1
      30  PRINT A

   If statement 30 is changed to PRINT A + 1, the program will output -32767, in- dicating that A actually has the value -32768 even though it cannot be dis- played. Furthermore, the following statements

      10  A = -32768
      20  INPUT A      where -32768 is typed in response to the input prompt

   will both result in error messages when executed.