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很少见的源码公开的msc51和z80的c编译器。

        is followed  by  a  comment.   For  example,  in  the  following
        statement:  

              label:    lda     abcd,x          ;Comment field 


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        SOURCE PROGRAM FORMAT



        the  tab  between lda and abcd terminates the operator field and
        defines the beginning of the operand field;  a  comma  separates
        the operands abcd and x;  and a semicolon terminates the operand
        field and defines the beginning of the comment field.   When  no
        comment  field  follows,  the operand field is terminated by the
        end of the source line.  


        1.2.1.4  Comment Field  - 

           The comment field begins with a semicolon and extends through
        the end of the line.  This field is optional and may contain any
        7-bit ascii character except null.  

           Comments  do not affect assembly processing or program execu-
        tion.  


        1.3  SYMBOLS AND EXPRESSIONS 


           This  section  describes the generic components of the ASxxxx
        assemblers:  the character set, the conventions observed in con-
        structing  symbols,  and  the use of numbers, operators, and ex-
        pressions.  


        1.3.1  Character Set 


           The following characters are legal in ASxxxx source programs: 

             1.  The  letters  A  through Z.  Both upper- and lower-case
                 letters are acceptable.  The assemblers are case sensi-
                 tive,  i.e.  ABCD and abcd are different symbols.  (The
                 assemblers can be made case insensitive by  recompiling
                 with the appropriate switches.) 

             2.  The digits 0 through 9 

             3.  The  characters . (period), $ (dollar sign), and _ (un-
                 derscore).  

             4.  The special characters listed in Tables 1 through 6.  


           Tables  1  through  6  describe  the various ASxxxx label and
        field terminators, assignment operators, operand separators, as-
        sembly, unary, binary, and radix operators.  


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        SYMBOLS AND EXPRESSIONS


        Table 1         Label Terminators and Assignment Operators 
        ---------------------------------------------------------------- 

                :   Colon               Label terminator.  

                ::  Double colon        Label  Terminator;   defines the
                                        label as a global label.  

                =   Equal sign          Direct assignment operator.  

                ==  Double equal        Direct assignment operator;  
                    sign                defines  the  symbol as a global
                                        symbol.  

        ---------------------------------------------------------------- 





        Table 2         Field Terminators and Operand Separators 
        ---------------------------------------------------------------- 

                    Tab                 Item or field terminator.  

                    Space               Item or field terminator.  

                ,   Comma               Operand field separator.  

                ;   Semicolon           Comment field indicator.  

        ---------------------------------------------------------------- 





        Table 3         Assembler Operators 
        ---------------------------------------------------------------- 

                #   Number sign         Immediate expression indicator. 

                .   Period              Current location counter.  

                (   Left parenthesis    Expression delimiter.  

                )   Right parenthesis   Expression delimeter.  

        ---------------------------------------------------------------- 


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        SYMBOLS AND EXPRESSIONS







        Table 4         Unary Operators 
        ---------------------------------------------------------------- 

                <   Left bracket        <FEDC   Produces  the lower byte
                                                value of the expression.
                                                (DC) 

                >   Right bracket       >FEDC   Produces  the upper byte
                                                value of the expression.
                                                (FE) 

                +   Plus sign           +A      Positive value of A 

                -   Minus sign          -A      Produces   the  negative
                                                (2's complement) of A.  

                ~   Tilde               ~A      Produces the 1's comple-
                                                ment of A.  

                '   Single quote        'D      Produces  the  value  of
                                                the character D.  

                "   Double quote        "AB     Produces the double byte
                                                value for AB.  

                \   Backslash           '\n     Unix style characters 
                                                \b, \f, \n, \r, \t 
                                     or '\001   or octal byte values.  

        ---------------------------------------------------------------- 







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        SYMBOLS AND EXPRESSIONS


        Table 5         Binary Operators 
        ---------------------------------------------------------------- 

                <<  Double          0800 << 4   Produces the 4 bit 
                    Left bracket                left-shifted   value  of
                                                0800.  (8000) 

                >>  Double          0800 >> 4   Produces the 4 bit 
                    Right bracket               right-shifted  value  of
                                                0800.  (0080) 

                +   Plus sign       A + B       Arithmetic      Addition
                                                operator.  

                -   Minus sign      A - B       Arithmetic   Subtraction
                                                operator.  

                *   Asterisk        A * B       Arithmetic   Multiplica-
                                                tion operator.   (signed
                                                16-bit) 

                /   Slash           A / B       Arithmetic      Division
                                                operator.        (signed
                                                16-bit quotient) 

                &   Ampersand       A & B       Logical AND operator.  

                |   Bar             A | B       Logical OR operator.  

                %   Percent sign    A % B       Modulus operator.  
                                                (16-bit value) 

                ^   Up arrow or     A ^ B       EXCLUSIVE OR operator.  
                    circumflex 

        ---------------------------------------------------------------- 







        THE ASSEMBLER                                          PAGE 1-10
        SYMBOLS AND EXPRESSIONS


        Table 6         Temporary Radix Operators 
        ---------------------------------------------------------------- 

                $%,   0b, 0B            Binary radix operator.  

                $&,   0o, 0O, 0q, 0Q    Octal radix operator.  

                $#,   0d, 0D            Decimal radix operator.  

                $$,   0h, 0H, 0x, 0X    Hexidecimal radix operator.  


                Potential  ambiguities arising from the use of 0b and 0d
                as temporary radix operators may be circumvented by pre-
                ceding  all  non-prefixed  hexidecimal  numbers with 00.
                Leading 0's are required in any  case  where  the  first
                hexidecimal  digit is abcdef as the assembler will treat
                the letter sequence as a label.  

        ---------------------------------------------------------------- 







        1.3.2  User-Defined Symbols 


           User-defined  symbols are those symbols that are equated to a
        specific value through a direct assignment statement  or  appear
        as  labels.  These symbols are added to the User Symbol Table as
        they are encountered during assembly.  

        The following rules govern the creation of user-defined symbols: 

             1.  Symbols  can  be  composed  of alphanumeric characters,
                 dollar signs ($),  periods  (.),  and  underscores  (_)
                 only.  

             2.  The  first  character  of a symbol must not be a number
                 (except in the case of local symbols).  

             3.  The  first eight characters of a symbol must be unique.
                 A symbol can be written  with  more  than  eight  legal
                 characters, but the ninth and subsequent characters are
                 ignored.  

             4.  Spaces and Tabs must not be embedded within a symbol.  




        THE ASSEMBLER                                          PAGE 1-11
        SYMBOLS AND EXPRESSIONS


        1.3.3  Local Symbols 


           Local  symbols are specially formatted symbols used as labels
        within a block of coding that has been delimited as a local sym-
        bol  block.   Local  symbols  are  of  the form n$, where n is a
        decimal integer from 0 to 255,  inclusive.   Examples  of  local
        symbols are:  

              1$ 
              27$ 
              138$ 
              244$ 

           The  range  of  a local symbol block consists of those state-
        ments between two normally constructed  symbolic  labels.   Note
        that a statement of the form:  

              ALPHA = EXPRESSION 

        is a direct assignment statement but does not create a label and
        thus does not delimit the range of a local symbol block.  

           Note that the range of a local symbol block may extend across
        program areas.  

           Local symbols provide a convenient means of generating labels
        for branch instructions and other such references  within  local
        symbol  blocks.   Using local symbols reduces the possibility of
        symbols with multiple definitions appearing within a  user  pro-
        gram.   In  addition,  the  use  of local symbols differentiates
        entry-point labels from local labels, since local labels  cannot
        be referenced from outside their respective local symbol blocks.
        Thus, local symbols of the same name can appear in  other  local
        symbol blocks without conflict.  Local symbols require less sym-
        bol table space than normal symbols.  Their use is recommended. 

           The  use of the same local symbol within a local symbol block
        will generate one or both of the m or p errors.  


        THE ASSEMBLER                                          PAGE 1-12
        SYMBOLS AND EXPRESSIONS


        Example of local symbols:  

                a:      ldx     #atable ;get table address
                        lda     #0d48   ;table length
                1$:     clr     ,x+     ;clear
                        deca
                        bne     1$