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Added the optional C keyword "volatile" to the __asm__ statement grammar.
It prevents the statement's Assembly code from being optimized (e.g., moved or removed). Optimization is disabled for that statement's entire function (other functions aren't affected).
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Greg King
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2c7ccca210
107
doc/cc65.sgml
107
doc/cc65.sgml
@ -2,8 +2,9 @@
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<article>
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<title>cc65 Users Guide
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<author><url url="mailto:uz@cc65.org" name="Ullrich von Bassewitz">
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<date>2015-05-26
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<author><url url="mailto:uz@cc65.org" name="Ullrich von Bassewitz">,<newline>
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<url url="mailto:gregdk@users.sf.net" name="Greg King">
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<date>2016-04-22
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<abstract>
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cc65 is a C compiler for 6502 targets. It supports several 6502 based home
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@ -15,7 +16,6 @@ computers like the Commodore and Atari machines, but it is easily retargetable.
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<!-- Begin the document -->
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<sect>Overview<p>
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cc65 was originally a C compiler for the Atari 8-bit machines written by
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@ -564,7 +564,7 @@ and the one defined by the ISO standard:
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that you must not mix pointers to those functions with pointers to
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user-written, cdecl functions (the calling conventions are incompatible).
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<p>
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<item> The <tt/volatile/ keyword doesn't have an effect. This is not as bad
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<item> The <tt/volatile/ keyword has almost no effect. That is not as bad
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as it sounds, since the 6502 has so few registers that it isn't
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possible to keep values in registers anyway.
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<p>
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@ -586,14 +586,14 @@ This cc65 version has some extensions to the ISO C standard.
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file. The syntax is
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<tscreen><verb>
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asm (<string literal>[, optional parameters]) ;
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asm [optional volatile] (<string literal>[, optional parameters]) ;
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</verb></tscreen>
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or
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<tscreen><verb>
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__asm__ (<string literal>[, optional parameters]) ;
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__asm__ [optional volatile] (<string literal>[, optional parameters]) ;
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</verb></tscreen>
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The first form is in the user namespace and is disabled if the <tt/-A/
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The first form is in the user namespace; and, is disabled if the <tt/-A/
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switch is given.
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There is a whole section covering inline assembler statements,
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@ -735,6 +735,7 @@ This cc65 version has some extensions to the ISO C standard.
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<p>
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<sect>Predefined macros<p>
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The compiler defines several macros at startup:
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@ -1224,39 +1225,44 @@ The compiler allows to insert assembler statements into the output file. The
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syntax is
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<tscreen><verb>
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asm (<string literal>[, optional parameters]) ;
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asm [optional volatile] (<string literal>[, optional parameters]) ;
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</verb></tscreen>
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or
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<tscreen><verb>
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__asm__ (<string literal>[, optional parameters]) ;
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__asm__ [optional volatile] (<string literal>[, optional parameters]) ;
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</verb></tscreen>
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<p>
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The first form is in the user namespace and is disabled by <tt><ref
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The first form is in the user namespace; and, is disabled by <tt><ref
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id="option--standard" name="--standard"></tt> if the argument is not <tt/cc65/.
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The asm statement may be used inside a function and on global file level. An
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inline assembler statement is a primary expression, so it may also be used as
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part of an expression. Please note however that the result of an expression
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containing just an inline assembler statement is always of type <tt/void/.
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The <tt/asm/ statement can be used only inside a function. Please note that
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the result of an inline assembler expression is always of type <tt/void/.
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The contents of the string literal are preparsed by the compiler and inserted
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into the generated assembly output, so that the can be further processed by
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the backend and especially the optimizer. For this reason, the compiler does
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only allow regular 6502 opcodes to be used with the inline assembler. Pseudo
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instructions (like <tt/.import/, <tt/.byte/ and so on) are <em/not/ allowed,
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The contents of the string literal are preparsed by the compiler; and, inserted
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into the generated assembly output, so that it can be processed further by
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the backend -- and, especially the optimizer. For that reason, the compiler does
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allow only regular 6502 opcodes to be used with the inline assembler. Pseudo
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instructions (like <tt/.import/, <tt/.byte/, and so on) are <em/not/ allowed,
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even if the ca65 assembler (which is used to translate the generated assembler
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code) would accept them. The builtin inline assembler is not a replacement for
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the full blown macro assembler which comes with the compiler.
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code) would accept them. The built-in inline assembler is not a replacement for
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the full-blown macro assembler which comes with the compiler.
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Note: Inline assembler statements are subject to all optimizations done by the
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compiler. There is currently no way to protect an inline assembler statement
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from being moved or removed completely by the optimizer. If in doubt, check
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the generated assembler output, or disable optimizations.
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compiler. There currently is no way to protect an inline assembler statement
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-- alone -- from being moved or removed completely by the optimizer. If in
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doubt, check the generated assembler output; or, disable optimizations (for
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that function).
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As a shortcut, you can put the <tt/volatile/ qualifier in your <tt/asm/
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statements. It will disable optimization for the functions in which those
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<tt/asm volatile/ statements sit. The effect is the same as though you put
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</#pragma optimize(push, off)/ above those functions, and </#pragma
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optimize(pop)/ below those functions.
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The string literal may contain format specifiers from the following list. For
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each format specifier, an argument is expected which is inserted instead of
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the format specifier before passing the assembly code line to the backend.
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the format specifier, before passing the assembly code line to the backend.
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<itemize>
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<item><tt/%b/ - Numerical 8-bit value
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@ -1269,33 +1275,33 @@ the format specifier before passing the assembly code line to the backend.
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<item><tt/%%/ - The % sign itself
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</itemize><p>
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Using these format specifiers, you can access C <tt/#defines/, variables or
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Using those format specifiers, you can access C <tt/#defines/, variables, or
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similar stuff from the inline assembler. For example, to load the value of
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a C <tt/#define/ into the Y register, one would use
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a C <tt/#define/ into the Y index register, one would use
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<tscreen><verb>
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#define OFFS 23
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__asm__ ("ldy #%b", OFFS);
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#define OFFS 23
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__asm__ ("ldy #%b", OFFS);
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</verb></tscreen>
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Or, to access a struct member of a static variable:
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<tscreen><verb>
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typedef struct {
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unsigned char x;
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unsigned char y;
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unsigned char color;
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} pixel_t;
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static pixel_t pixel;
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__asm__ ("ldy #%b", offsetof(pixel_t, color));
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__asm__ ("lda %v,y", pixel);
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typedef struct {
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unsigned char x;
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unsigned char y;
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unsigned char color;
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} pixel_t;
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static pixel_t pixel;
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__asm__ ("ldy #%b", offsetof(pixel_t, color));
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__asm__ ("lda %v,y", pixel);
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</verb></tscreen>
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<p>
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The next example shows how to use global variables to exchange data between C
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an assembler and how to handle assembler jumps:
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and assembler; and, how to handle assembler jumps:
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<tscreen><verb>
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unsigned char globalSubA, globalSubB, globalSubResult;
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static unsigned char globalSubA, globalSubB, globalSubResult;
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/* return a-b, return 255 if b>a */
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unsigned char sub (unsigned char a, unsigned char b)
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@ -1314,19 +1320,19 @@ an assembler and how to handle assembler jumps:
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</verb></tscreen>
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<p>
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Arrays can also be accessed:
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Arrays also can be accessed:
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<tscreen><verb>
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unsigned char globalSquareTable[] = {
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static const unsigned char globalSquareTable[] = {
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0, 1, 4, 9, 16, 25, 36, 49, 64, 81,
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100, 121, 144, 169, 196, 225
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};
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unsigned char globalSquareA, globalSquareResult;
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static unsigned char globalSquareA, globalSquareResult;
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/* return a*a for a<16, else 255 */
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unsigned char square (unsigned char a)
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{
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if (a>15){
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if (a > 15) {
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return 255;
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}
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globalSquareA = a;
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@ -1339,28 +1345,30 @@ Arrays can also be accessed:
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<p>
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Note: Do not embed the assembler labels that are used as names of global
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variables or functions into your asm statements. Code like this
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variables or functions into your <tt/asm/ statements. Code such as this:
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<tscreen><verb>
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int foo;
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int bar () { return 1; }
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__asm__ ("lda _foo"); /* DON'T DO THAT! */
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int bar (void) { return 1; }
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...
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__asm__ ("lda _foo"); /* DON'T DO THAT! */
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...
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__asm__ ("jsr _bar"); /* DON'T DO THAT EITHER! */
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</verb></tscreen>
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<p>
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may stop working if the way, the compiler generates these names is changed in
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a future version. Instead use the format specifiers from the table above:
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might stop working if the way that the compiler generates those names is changed in
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a future version. Instead, use the format specifiers from the table above:
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<tscreen><verb>
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__asm__ ("lda %v", foo); /* OK */
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__asm__ ("lda %v", foo); /* OK */
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...
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__asm__ ("jsr %v", bar); /* OK */
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</verb></tscreen>
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<p>
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<sect>Implementation-defined behavior<p>
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This section describes the behavior of cc65 when the standard describes the
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@ -1434,4 +1442,3 @@ freely, subject to the following restrictions:
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</enum>
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</article>
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@ -41,12 +41,14 @@
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/* cc65 */
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#include "asmlabel.h"
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#include "codegen.h"
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#include "codeseg.h"
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#include "datatype.h"
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#include "error.h"
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#include "expr.h"
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#include "function.h"
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#include "litpool.h"
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#include "scanner.h"
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#include "segments.h"
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#include "stackptr.h"
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#include "symtab.h"
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#include "asmstmt.h"
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@ -422,6 +424,15 @@ void AsmStatement (void)
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/* Skip the ASM */
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NextToken ();
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/* An optional volatile qualifier disables optimization for
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** the entire function [same as #pragma optimize(push, off)].
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*/
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if (CurTok.Tok == TOK_VOLATILE) {
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/* Don't optimize the Current code Segment */
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CS->Code->Optimize = 0;
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NextToken ();
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}
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/* Need left parenthesis */
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if (!ConsumeLParen ()) {
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return;
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