mdk_tut.texi

汇编语言编程源代码

compiled program (that is, the address at which the virtual MIX machine
must begin fetching instructions after loading the program). It is also
very common (although not mandatory) to include at least an @code{ORIG}
directive to mark the initial value of the assembler's location counter
(remember that it stores the address associated with each compiled MIX
instruction). Thus, a minimal MIXAL program would be

@example
          ORIG  2000    set the initial compilation adress
          NOP           this instruction will be loaded at adress 2000
          HLT           and this one at address 2001
          END   2000    end of program; start at address 2000
this line is not parsed by the assembler
@end example
@noindent
The assembler will generate two binary instructions (@code{NOP} (@w{+ 00
00 00 00 00}) and @code{HLT} (+ 00 00 02 05)), which will be loaded at
addresses 2000 and 2001. Execution of the program will begin at address
2000. Every MIXAL program should also include a @code{HLT} instruction,
which will mark the end of program execution (but not of program
compilation). 

The @code{EQU} directive allows the definition of symbolic names for
specific values. For instance, we could rewrite the above program as
follows:

@example
START     EQU   2000
          ORIG  START
          NOP
          HLT
          END   START
@end example
@noindent
which would give rise to the same compiled code. Symbolic constants (or
symbols, for short) can also be implicitly defined placing them in the
@code{LABEL} field of a MIXAL instruction: in this case, the assembler
assigns to the symbol the value of the location counter before compiling
the line. Hence, a third way of writing our trivial program is

@example
          ORIG  2000
START     NOP
          HLT
          END   START
@end example

The @code{CON} directive allows you to directly specify the contents of
the memory address pointed by the location counter. For instance, when
the assembler encounters the following code snippet

@example
          ORIG  1150
          CON   -1823473
@end example
@noindent
it will assign to the memory cell number 1150 the contents @w{- 00 06 61
11 49} (which corresponds to the decimal value -1823473). 

Finally, the @code{ALF} directive let's you specify the memory contents
as a set of five (quoted) characters, which are translated by the
assembler to their byte values, conforming in that way the binary word
that is to be stored in the corresponding memory cell. This directive
comes in handy when you need to store printable messages in a memory
address, as in the following example:

@example
          OUT  MSG       MSG is not yet defined here (future reference)
MSG       ALF  "THIS "   MSG gets defined here
          ALF  "IS A "
          ALF  "MESSA"
          ALF  "GE.  "
@end example
@noindent
The above snippet also shows the use of a @dfn{future reference}, that
is, the usage of a symbol (@code{MSG} in the example) prior of its actual
definition. The MIXAL assembler is able to handle future references
subject to some limitations which are described in the following section
(@pxref{Expressions}).

@cindex comments

Any line starting with an asterisk is treated as a comment and ignored
by the assembler.

@example
* This is a comment: this line is ignored.
    * This line is an error: * must be in column 1.
@end example

As noted in the previous section, comments can also be located after the
@code{OPERAND} field of an instruction, separated from it by white
space, as in

@example
LABEL     LDA   100  This is also a comment
@end example

@node Expressions, W-expressions, MIXAL directives, MIXAL
@comment  node-name,  next,  previous,  up
@subsection Expressions
@cindex operator
@cindex binary operator
@cindex unary operator
The @code{ADDRESS}, @code{INDEX} and @code{MOD} fields of a MIXAL
instruction can be expressions, formed by numbers, identifiers and
binary operators (@code{+ - * / // :}). @code{+} and @code{-} can also
be used as unary operators. Operator precedence is from left to right:
there is no other operator precedence rule, and parentheses cannot be
used for grouping. A stand-alone asterisk denotes the current memory
location; thus, for instance,

@example
     4+2**
@end example

@noindent
evaluates to 6 (4 plus 2) times the current memory location. White space
is not allowed within expressions.

The special binary operator @code{:} has the same meaning as in fspecs,
i.e.,

@example
A:B = 8*A + B
@end example
@noindent
while @code{A//B} stands for the quotient of the ten-byte number @w{@code{A} 00
00 00 00 00} (that is, A right-padded with 5 null bytes or, what amounts
to the same, multiplied by 64 to the fifth power) divided by
@code{B}. Sample expressions are:

@example
18-8*3 = 30
14/3 = 4
1+3:11 = 4:11 = 43
1//64 = (01 00 00 00 00)/(00 00 00 01 00) = (01 00 00 00 00)
@end example
@noindent
Note that all MIXAL expressions evaluate to a MIX word (by definition).

All symbols appearing within an expression must be previously defined. Future
references are only allowed when appearing standalone (or modified by
an unary operator) in the @code{ADDRESS} part of a MIXAL instruction,
e.g.

@example
* OK: stand alone future reference 
         STA  -S1(1:5)
* ERROR: future reference in expression
         LDX  2-S1
S1       LD1  2000
@end example

@node W-expressions, Local symbols, Expressions, MIXAL
@comment  node-name,  next,  previous,  up
@subsection W-expressions
@cindex w-expressions

Besides expressions, as described above (@pxref{Expressions}), the MIXAL
assembler is able to handle the so called @dfn{w-expressions} as the
operands of the directives @code{ORIG}, @code{EQU}, @code{CON} and
@code{END} (@pxref{MIXAL directives}). The general form of a
w-expression is the following:

@example
     WEXP = EXP[(EXP)][,WEXP]
@end example
@noindent
where @code{EXP} stands for an expression and square brackets denote
optional items. Thus, a w-expression is made by an expression, followed
by an optional expression between parenthesis, followed by any number
of similar constructs separated by commas. Sample w-expressions are:

@example
2000
235(3)
S1+3(S2),3000
S1,S2(3:5),23
@end example

W-expressions are evaluated from left to right as follows:

@itemize
@item
Start with an accumulated result @samp{w} equal to 0.
@item
Take the first expression of the comma-separated list and evaluate
it. For instance, if the w-expression is @samp{S1+2(2:4),2000(S2)}, we
evaluate first @samp{S1+2}; let's suppose that @samp{S1} equals
265230: then @samp{S1+2 = 265232 = + 00 01 00 48 16}.
@item
Evaluate the expression within parenthesis, reducing it to an f-spec
of the form @samp{L:R}. In our previous example, the expression
between parenthesis already has the desired form: 2:4.
@item
Substitute the bytes of the accumulated result @samp{w} designated by
the f-spec using those of the previous expression value. In our sample,
@samp{w = + 00 00 00 00 00}, and we must substitute bytes 2, 3 and 4 of
@samp{w} using values from 265232. We need 3 bytes, and we take the
least significant ones: 00, 48, and 16, and insert them in positions
2, 3 and 4 of @samp{w}, obtaining @samp{w = + 00 00 48 16 00}.
@item
Repeat this operation with the remaining terms, acting on the new
value of @samp{w}. In our example, if, say, @samp{S2 = 1:1}, we must
substitute the first byte of @samp{w} using one byte (the least
significant) from 2000, that is, 16 (since 2000 = + 00 00 00 31 16)
and, therefore, we obtain @samp{w = + 16 00 48 16 00}; summing up, we
have obtained @samp{265232(1:4),2000(1:1) = + 16 00 48 16 00 =
268633088}.
@end itemize

As a second example, in the w-expression
@example
1(1:2),66(4:5)
@end example
@noindent
we first take two bytes from 1 (00 and 01) and store them as bytes 1 and
2 of the result (obtaining @w{@samp{+ 00 01 00 00 00}}) and, afterwards,
take two bytes from 66 (01 and 02) and store them as bytes 4 and 5 of
the result, obtaining @w{@samp{+ 00 01 00 01 02}} (262210). The process
is repeated for each new comma-separated example. For instance:

@example
1(1:1),2(2:2),3(3:3),4(4:4) = 01 02 03 04 00
@end example

As stated before, w-expressions can only appear as the operands of MIXAL
directives taking a constant value (@code{ORIG}, @code{EQU}, @code{CON}
and @code{END}). Future references are @emph{not} allowed within
w-expressions (i.e., all symbols appearing in a w-expression must be
defined before it is used).

@node Local symbols, Literal constants, W-expressions, MIXAL
@comment  node-name,  next,  previous,  up
@subsection Local symbols
@cindex local symbols

Besides user defined symbols, MIXAL programmers can use the so called
@dfn{local symbols}, which are symbols of the form @code{[1-9][HBF]}. A
local symbol @code{nB} refers to the address of the last previous
occurrence of @code{nH} as a label, while @code{nF} refers to the next
@code{nH} occurrence. Unlike user defined symbols, @code{nH} can appear
multiple times in the @code{LABEL} part of different MIXAL
instructions. The following code shows an instance of local symbols'
usage:

@example
* line 1
1H    LDA  100
* line 2: 1B refers to address of line 1, 3F refers to address of line 4
      STA  3F,2(1B//2)
* line 3: redefinition of 1H
1H    STZ
* line 4: 1B refers to address of line 3
3H    JMP  1B
@end example

Note that a @code{B} local symbol never refers to a definition in its
own line, that is, in the following program:

@example
		ORIG 1999
ST		NOP
3H		EQU 69
3H		ENTA 3B  local symbol 3B refers to 3H in previous line
		HLT
		END ST
@end example
@noindent
the contents of @samp{rA} is set to 69 and @emph{not} to 2001. An
specially tricky case occurs when using local symbols in conjunction
with @code{ORIG} pseudoinstructions. To wit@footnote{The author wants to
thank Philip E. King for pointing these two special cases of local
symbol usage to him.},

@example
		ORIG 1999 
ST		NOP
3H		CON 10
		ENT1 *
		LDA 3B
** rI1 is 2001, rA is 10.  So far so good!
3H		ORIG 3B+1000
** at this point 3H equals 2003
** and the location counter equals 3000.
		ENT2 *
		LDX 3B
** rI2 contains 3000, rX contains 2003.
		HLT
		END ST
@end example

@node Literal constants,  , Local symbols, MIXAL
@comment  node-name,  next,  previous,  up
@subsection Literal constants
@cindex literal constants

MIXAL allows the introduction of @dfn{literal constants}, which are
automatically stored in memory addresses after the end of the program by
the assembler. Literal constants are denoted as @code{=wexp=}, where
@code{wexp} is a w-expression (@pxref{W-expressions}). For instance, the
code

@example
L         EQU   5
          LDA   =20-L=
@end example

causes the assembler to add after the program's end an instruction
with contents 15 (@samp{20-L}), and to assemble the above code as the
instruction @w{@code{ LDA a}}, where @code{a} stands for the address
in which the value 15 is stored. In other words, the compiled code is
equivalent to the following:

@example
L         EQU  5
          LDA  a
@dots{}
a         CON  20-L    
          END  start
@end example