TB51 is an interpreter program for a language based on the "Palo Alto tiny BASIC" specifications modified for Intel's MCS-51 family of single-chip microcontrollers.


An 8051 or 8751 with the interpreter program in the ROM or EPROM, CRT, and level shifters on the serial I/O pins.  External program and/or data memory may be added to develop and execute larger programs.  The interpreter program occupies 4K (3K if the included BASIC example program is eliminated), and can operate in a limited manner using only the 128 bytes of RAM available on the 8051.  Full use is possible only with the addition of external RAM.


ASM-51     V 2.0

Intel MCS-51 Tiny BASIC

In 1976, Dr. Dobb's Journal of Computer Calisthenics and Orthodontia proposed a subset of the BASIC language which would be appropriate for small microcomputer systems. One early version of the language (dubbed "Palo Alto tiny BASIC") was defined and implemented by Dr. Li-Chen Wang, and was later adapted for the 8085 and 8088 MPUs.

TB51 is an interpreter program for a language based on the "Palo Alto tiny BASIC" specification which has been implemented for the Intel MCS-51 family of single-chip microcontrollers.  Extensions to the language facilitate hexadecimal arithmetic, logical operations, and bit manipulation for controller-oriented applications.  Rudimentary system monitor capabilities are also provided.  Some minor changes have been made to make TB51 more closely resemble BASIC-80, PLM-80, and the ISIS-II operating system.

TB51 operates in a number of modes to support various hardware configurations.  The simplest configuration requires only an 8051 or 8751 single-chip microcomputer containing the interpreter program in on-chip ROM/EPROM, a CRT, and level shifters on the microcomputer serial I/O pins.  External program and/or data memory may be added by the user to develop and execute larger programs.  The following discussion defines the operation of TB51 initially assuming there is at least one page (256 bytes) of external RAM provided, describes the difference between the DECIMAL and HEX operating modes, explains the additional system monitor capabilities, and lists the special considerations for true single-chip operation.

TB51 has an auto-baud detection algorithm.  Following system reset, typing either a space character or a lower-case "c" once or twice will initialize the baud rate.  Which character works, and how many times it must be typed, is a function of the baud rate of the connected terminal, and of the system clock rate.  Sometimes other characters work as well.

If the RxD serial I/O pin is jumpered to ground, TB51 will automatically begin execution of a program stored in external PROM following a reset, and the auto-baud detection will be skipped.



All numbers are signed integers and must be between -32767 and 32767.
There are 26 variables denoted by the letters A through Z.  These are represented internally as 16-bit, two's-complement integers.
There are 2 functions:
ABS(X) gives the absolute value of X.
RND(X) gives a random number between 1 and X (inclusive).
All operations are performed to 16 bits of precision.  Arithmetic operations which overflow 15 bits of magnitude will produce an error message.  The arithmetic operators are:
Operator Function Comment
+ addition
- subtraction
* multiplication
/ integer division note 14/5 = 2
MOD remainder from division 14 MOD 5 = 4
The logical operators are:
Operator Function Comment
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:
Operator Function
> greater than.
< less than.
= equal to.
<> not equal to.
>= greater than or equal to.
<= less than or equal to.
Arithmetic and logical 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 are formed with numbers, variables, and functions with arithmetic, logical, and compare operators between them.  Operations are performed with four levels of precedence.  The first step is to perform unary operations (+, -, NOT), then multiplicative operations (*, /, MOD, AND), additive operations (+, -, OR, XOR), and finally, relational operations (>, <, =, <>, >=, <=).

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 prescribed 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.

Commands may be entered in upper or lower case, but will be converted to upper case.  All the commands described later can be used as direct commands except the following three; they should only be used as direct commands and not as part of a statement:


Will start to execute the program starting at the lowest statement number.
Will print out all the statements in numerical order.
LIST 120
Will print out all the statements in numerical order starting at statement 120.
Will delete all statements and set variables A-Z to zero.


You may use blanks freely, except that numbers, command key words, and function names can not have embedded 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 variable 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
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


A statement consists of a statement number between 1 and 32767 followed by one or more commands.  Commands in the same statement are separated by a colon ":".  "GOTO", "STOP", "FOR", and "RETURN" commands must be the last command in any given statement.


Tiny Basic commands are listed below with examples.  Remember that commands can be concatenated with 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 TB51

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


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 automatically 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


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>.


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:".


The strings and semicolons have the same effect as in "PRINT".

IF Command


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 statement.  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.


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.


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.


The name of the variable (X) is checked with that of the most recent FOR command.  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 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.

END Command


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


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

There are only three error conditions in TINY BASIC.

WHAT? means it does not understand you.
    Example:   where PRINT is mistyped

    210  PTINT "THIS"

    When this line is executed, TB51 will halt and print "WHAT?".

HOW? means is understands you but does not know how to do it.
    Example:   where B * C is greater than 32767

    310  LET A = B * C + 2

    Example:   where 412 does not exist

    410  GOTO 412

    When these lines are executed, TB51 will halt and print "HOW?".

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.


In interactive command or line insertion mode, line editing capabilities similar to ISIS-II are allowed.  If you notice an error in typing before you hit CR, you can delete previously typed characters with the rubout key (ASCII 127).  TB51 will echo a BKSP-SP-BKSP for each rubout.  A line may be retyped by hitting Control-R, or cancelled by typing Control-X.  A BELL will be echoed when the input line buffer is full (32 characters).

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

Each BASIC statement is stored in the program buffer using the following format.  At the beginning of the line is a two-byte field giving the 16-bit binary value of the statement number, high-order byte first.  Thus line number 10 will be stored as 00H, 0AH.  These numbers may range from 0000H to 7FFFH.  Then comes an arbitrary number of bytes giving the ASCII codes for the characters comprising any legal sequence of commands.  At the end of each line comes the byte 0DH, the ASCII representation for a carriage return.  Following the CR after the last source statement, in place of what would be the high-order byte of the following statement number, is the data byte 0FFH.


TB51 provides for BASIC programs to be buffered in three different address spaces.  Following a reset or the RAM command, programs entered from the keyboard will be buffered in up to 4K bytes of external RAM, beginning at location 2000H.  By typing the command ROM, the RAM buffer will be disabled and a BASIC program included with the TB51 4K byte ROM code will be activated.  Thereafter this program can be LISTed, RUN, terminated by Control-C, and so forth, but obviously cannot be modified from the keyboard.  Similarly, a user-written BASIC program stored in an external PROM or EPROM may be activated by typing PROM. This program is assumed to begin at location 1080H, and may be up to 60K bytes long.  It is also assumed that this PROM be addressed as 8051 program memory.

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 assembly 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 addition to these features, the following monitor-like capabilities are provided. Note that these capabilities are fully compatible with, and may be freely interchanged with, the commands and statements described thus far.


In addition to the 16-bit BASIC variables denoted A through Z, various 8051 system resources may be read or written as 8-bit or one-bit positive variables. Anywhere an alphabetic variable name is allowed, these variables may be accessed by a keyword representing a memory space followed by a number, simple variable, or parenthesized expression indicating the address of the byte or bit desired. The addresses interpreted by the DBYTE and RBIT commands are the same as those used by the 8051 CPU for direct byte or bit addressing.  Addresses between 0 and 127 access the on-chip RAM array; between 128 and 255 access bytes or bits in the special function register space.  (Only the low-order eight bits of the specified address are significant.)  The decimal addresses of accessible registers are:
P0 128
P1 144
P2 160
P3 176
TCON 136
TMOD 137
TL0 138
TL1 139
TH0 140
TH1 141
SCON 152
SBUF 153
IE 168
IP 184
Attempting to read or write registers other than those above will cause an error message.


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.


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 registers and initialize sequential memory locations using a syntax similar to that of Intel's In-Circuit Emulators.


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.


In the DECIMAL operating mode, variables are treated as two's-complement signed decimal integers.  An error message is generated whenever the magnitude of a number or expression exceeds 32767.  DECIMAL is the default radix following a reset or the NEW command.

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 hexadecimal digits, provided the first digit is between 0 and 9.  If more than four digits are entered, the higher order bits are ignored.  In the interest of compatibility 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 assumed.  Numbers will be printed out in HEX mode as two to four hex digits followed 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

CALL 8192 will call an 8051 machine language subroutine assumed to be present starting at address 2000H.  This routine will execute until a RET instruction (machine code 22H) is encountered, at which time execution of the BASIC program will continue with the following 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

As implied at the beginning of this discussion, the TB51 interpreter may be used even in systems with no external RAM.  However, this naturally requires certain limitations.

1.) With no program line buffering, BASIC and machine language programs cannot 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 variables (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.

Notice that these restrictions do not preclude the execution of programs stored in internal ROM or external PROM, provided they are written to use only variables A through L.


TB51 release V2.2 (as identified in the sign-on message) contains the following known bugs.  They should not seriously affect the program's usability.

RND : 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.

FOR-NEXT : 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.

-32768 :  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

If statement 30 is changed to PRINT A + 1, the program will output -32767, indicating that A actually has the value -32768 even though it cannot be displayed.  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.

Copyright © Madis Kaal 2000-