asmlnk.doc

很少见的源码公开的msc51和z80的c编译器。

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


        1.3.4  Current Location Counter 


           The  period  (.) is the symbol for the current location coun-
        ter.  When used in the operand  field  of  an  instruction,  the
        period   represents  the  address  of  the  first  byte  of  the
        instruction:  

                AS:     ldx     #.      ;The period (.) refers to
                                        ;the address of the ldx
                                        ;instruction.

           When  used  in  the  operand field of an ASxxxx directive, it
        represents the address of the current byte or word:  

                QK = 0
        
                .word   0xFFFE,.+4,QK   ;The operand .+4 in the .word
                                        ;directive represents a value
                                        ;stored in the second of the
                                        ;three words during assembly.

           If  we  assume  the  current  value of the program counter is
        0H0200, then during assembly, ASxxxx  reserves  three  words  of
        storage  starting  at  location 0H0200.  The first value, a hex-
        idecimal constant FFFE, will be stored at location 0H0200.   The
        second  value  represented  by  .+4  will  be stored at location
        0H0202, its value will be 0H0206 ( = 0H0202  +  4).   The  third
        value  defined  by  the  symbol  QK  will  be placed at location
        0H0204.  

           At the beginning of each assembly pass, ASxxxx resets the lo-
        cation counter.  Normally, consecutive memory locations are  as-
        signed  to  each  byte  of  object code generated.  However, the
        value of the location counter can be changed  through  a  direct
        assignment statement of the following form:  



        THE ASSEMBLER                                          PAGE 1-13
        SYMBOLS AND EXPRESSIONS


              . = . + expression 


           The  new  location  counter can only be specified relative to
        the current location counter.  Neglecting to specify the current
        program  counter  along with the expression on the right side of
        the assignment operator will generate the (.) error.   (Absolute
        program areas may use the .org directive to specify the absolute
        location of the current program counter.) 

        The following coding illustrates the use of the current location
        counter:  

                .area   CODE1   (ABS)   ;program area CODE1
                                        ;is ABSOLUTE
        
                .org    0H100           ;set location to
                                        ;0H100 absolute
        
        num1:   ldx     #.+0H10         ;The label num1 has
                                        ;the value 0H100.
                                        ;X is loaded with
                                        ;0H100 + 0H10
        
                .org    0H130           ;location counter
                                        ;set to 0H130
        
        num2:   ldy     #.              ;The label num2 has
                                        ;the value 0H130.
                                        ;Y is loaded with
                                        ;value 0H130.
        
        
                .area   CODE2   (REL)   ;program area CODE2
                                        ;is RELOCATABLE
        
                . = . + 0H20            ;Set location counter
                                        ;to relocatable 0H20 of
                                        ;the program section.
        
        num3:   .word   0               ;The label num3 has
                                        ;the value
                                        ;of relocatable 0H20.
        
                . = . + 0H40            ;will reserve 0H40
                                        ;bytes of storage as will
                .blkb   0H40            ;or
                .blkw   0H20

           The  .blkb  and .blkw directives are the preferred methods of
        allocating space.  



        THE ASSEMBLER                                          PAGE 1-14
        SYMBOLS AND EXPRESSIONS


        1.3.5  Numbers 


           ASxxxx  assumes that all numbers in the source program are to
        be interpreted in decimal radix unless otherwise specified.  The
        .radix  directive  may  be used to specify the default as octal,
        decimal, or hexidecimal.  Individual numbers can  be  designated
        as  binary, octal, decimal, or hexidecimal through the temporary
        radix prefixes shown in table 6.  

           Negative  numbers  must be preceeded by a minus sign;  ASxxxx
        translates such numbers into two's  complement  form.   Positive
        numbers may (but need not) be preceeded by a plus sign.  

           Numbers are always considered to be absolute values, therefor
        they are never relocatable.  


        1.3.6  Terms 


           A  term is a component of an expression and may be one of the
        following:  


             1.  A number.  

             2.  A symbol:  
                 1.  A  period (.) specified in an expression causes the
                     current location counter to be used.  
                 2.  A User-defined symbol.  
                 3.  An undefined symbol is assigned a value of zero and
                     inserted in the User-Defined symbol table as an un-
                     defined symbol.  

             3.  A single quote followed by a single ascii character, or
                 a double quote followed by two ascii characters.  

             4.  An  expression enclosed in parenthesis.  Any expression
                 so enclosed is evaluated and reduced to a  single  term
                 before  the remainder of the expression in which it ap-
                 pears is evaluated.  Parenthesis, for example,  may  be
                 used  to  alter the left-to-right evaluation of expres-
                 sions, (as in A*B+C versus A*(B+C)), or to apply a  un-
                 ary operator to an entire expression (as in -(A+B)).  

             5.  A unary operator followed by a symbol or number.  





        THE ASSEMBLER                                          PAGE 1-15
        SYMBOLS AND EXPRESSIONS


        1.3.7  Expressions 


           Expressions  are  combinations  of  terms  joined together by
        binary operators.  Expressions reduce to a  16-bit  value.   The
        evaluation  of  an  expression includes the determination of its
        attributes.  A resultant expression value may be  one  of  three
        types  (as  described  later in this section):  relocatable, ab-
        solute, and external.  

        Expressions are evaluate with an operand hierarchy as follows:  

                *       /       %       multiplication,
                                        division, and
                                        modulus first.
        
                +       -               addition and
                                        subtraction second.
        
                <<      >>              left shift and
                                        right shift third.
        
                ^                       exclusive or fourth.
        
                &                       logical and fifth.
        
                |                       logical or last
        
                except that unary operators take precedence over binary
                operators.


           A  missing  or  illegal  operator  terminates  the expression
        analysis, causing error codes (o) and/or  (q)  to  be  generated
        depending upon the context of the expression itself.  

           At assembly time the value of an external (global) expression
        is equal to the value of the absolute part of  that  expression.
        For  example,  the expression external+4, where 'external' is an
        external symbol, has the value of 4.  This expression,  however,
        when  evaluated  at link time takes on the resolved value of the
        symbol 'external', plus 4.  

           Expressions,  when  evaluated  by  ASxxxx,  are  one of three
        types:  relocatable, absolute, or external.  The following  dis-
        tinctions are important:  

             1.  An  expression is relocatable if its value is fixed re-
                 lative to the base address of the program area in which
                 it appears;  it will have an offset value added at link
                 time.  Terms that contain labels defined in relocatable
                 program   areas   will   have   a   relocatable  value;


        THE ASSEMBLER                                          PAGE 1-16
        SYMBOLS AND EXPRESSIONS


                 similarly, a period (.) in a relocatable program  area,
                 representing  the value of the current program location
                 counter, will also have a relocatable value.  

             2.  An  expression  is  absolute if its value is fixed.  An
                 expression whose terms are numbers and ascii characters
                 will  reduce  to  an absolute value.  A relocatable ex-
                 pression or term minus a relocatable term,  where  both
                 elements  being  evaluated  belong  to the same program
                 area, is an absolute expression.  This is because every
                 term  in  a  program area has the same relocation bias.
                 When one term is subtracted from the other the  reloca-
                 tion bias is zero.  

             3.  An  expression is external (or global) if it contains a
                 single global reference (plus or minus an absolute  ex-
                 pression  value) that is not defined within the current
                 program.  Thus, an external  expression  is  only  par-
                 tially  defined following assembly and must be resolved
                 at link time.  



        1.4  GENERAL ASSEMBLER DIRECTIVES 


           An  ASxxxx  directive  is placed in the operator field of the
        source line.  Only one directive is  allowed  per  source  line.
        Each  directive  may  have  a blank operand field or one or more
        operands.  Legal operands differ with each directive.  


        1.4.1  .module Directive 

        Format:  

                .module string 

           The .module directive causes the string to be included in the
        assemblers output file as an identifier for this particular  ob-
        ject  module.   The  string  may  be  from  1 to 8 characters in
        length.  Only one identifier is allowed  per  assembled  module.
        The  main use of this directive is to allow the linker to report
        a modules' use of undefined symbols.  At link time all undefined
        symbols  are  reported  and  the  modules  referencing  them are
        listed.  




        THE ASSEMBLER                                          PAGE 1-17
        GENERAL ASSEMBLER DIRECTIVES


        1.4.2  .title Directive 

        Format:  

                .title  string 

           The .title directive provides a character string to be placed
        on the second line of each page during listing.  


        1.4.3  .sbttl Directive 

        Format:  

                .sbttl  string 

           The .sbttl directive provides a character string to be placed
        on the third line of each page during listing.  


        1.4.4  .page Directive 

        Format:  

                .page 

           The .page directive causes a page ejection with a new heading
        to be printed.  The new page occurs after the next line  of  the
        source  program is processed, this allows an immediately follow-
        ing .sbttl directive to appear  on  the  new  page.   The  .page
        source  line will not appear in the file listing.  Paging may be
        disabled by invoking the -p directive.  


        1.4.5  .byte and .db Directives 

        Format:  

                .byte   exp             ;Stores the binary value
                .db     exp             ;of the expression in the
                                        ;next byte.
        
                .byte   exp1,exp2,expn  ;Stores the binary values
                .db     exp1,exp2,expn  ;of the list of expressions
                                        ;in successive bytes.
        
        where:  exp,    represent expressions that will be
                exp1,   truncated to 8-bits of data.
                .       Each expression will be calculated