Assembler Modes and Pragmas

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# 3.10. Assembler Modes and Pragmas

There are a number of options that affect the way assembly is performed. Some of these options can only be specified on the command line because they determine something absolute about the assembly process. These include such things as the output target. Other things may be switchable during the assembly process. These are known as pragmas and are, by definition, not portable between assemblers.

LWASM supports a number of pragmas that affect code generation or otherwise affect the behaviour of the assembler. These may be specified by way of a command line option or by assembler directives. The directives are as follows.

PRAGMA `pragma[,...]`

  Specifies that the assembler should bring into force all `pragma`s specified. Any unrecognized pragma will cause an assembly error. The new pragmas will take effect immediately. This directive should be used when the program will assemble incorrectly if the pragma is ignored or not supported.

*PRAGMA `pragma[,...]`

  This is identical to the PRAGMA directive except no error will occur with unrecognized or unsupported pragmas. This directive, by virtue of starting with a comment character, will also be ignored by assemblers that do not support this directive. Use this variation if the pragma is not required for correct functioning of the code.

*PRAGMAPUSH `pragma[,...]`

  This directive saves the current state of the specified pragma(s) for later retrieval. See discussion below for more information.

  This directive will not throw any errors for any reason.

*PRAGMAPOP `pragma[,...]`

  This directive restores the previously saved state of the specified pragma(s). See discussion below for more information.

  This directive will not throw any errors for any reason.

Each pragma supported has a positive version and a negative version. The positive version enables the pragma while the negative version disables it. The negatitve version is simply the positive version with "no" prefixed to it. For instance, "pragma" vs. "nopragma". When only one version is listed below, its opposite can be obtained by prepending "no" if it is not present or removing "no" from the beginning if it is present.

Pragmas are not case sensitive.

6800compat

  When in force, this pragma enables recognition of various compatibility instructions useful when assembling 6800 code. These compatibility instructions are assembled into equivalent 6809 instructions. This mode also includes several analogous instructions which are not strictly 6800 instructions but allow the similar style to be applied to 6809 specific features.

  Technically, a compliant 6809 assembler must recognize these instructions by default since Motorola advertised the 6809 as being source compatible with the 6800. However, most source code does not require this compatibility and LWASM itself did not support these instructions prior to version 4.11 so this mode is disabled by default.

6809

  This pragma allows you to mark a section of code as 6809-only. In ths mode, the assembler will throw an error if any 6309 instructions are used.

6309

  This pragma enables the use of 6309 instructions and disables any 6809 specific instructions. It also changes the cycle count listing output (if selected) to display 6309 timings.

6809conv, 6309conv

  These pragmas enable convenience instructions extending the 6809 and 6309 instruction sets respectively. For more information, see Section 3.11.

index0tonone

  When in force, this pragma enables an optimization affecting indexed addressing modes. When the offset expression in an indexed mode evaluates to zero but is not explicity written as 0, this will replace the operand with the equivalent no offset mode, thus creating slightly faster code. Because of the advantages of this optimization, it is enabled by default.

cescapes

  This pragma will cause strings in the FCC, FCS, and FCN pseudo operations to have C-style escape sequences interpreted. The one departure from the official spec is that unrecognized escape sequences will return either the character immediately following the backslash or some undefined value. Do not rely on the behaviour of undefined escape sequences.

importundefexport

  This pragma is only valid for targets that support external references. When in force, it will cause the EXPORT directive to act as IMPORT if the symbol to be exported is not defined. This is provided for compatibility with the output of gcc6809 and should not be used in hand written code. Because of the confusion this pragma can cause, it is disabled by default.

undefextern

  This pragma is only valid for targets that support external references. When in force, if the assembler sees an undefined symbol on the second pass, it will automatically define it as an external symbol. This automatic definition will apply for the remainder of the assembly process, even if the pragma is subsequently turned off. Because this behaviour would be potentially surprising, this pragma defaults to off.

  The primary use for this pragma is for projects that share a large number of symbols between source files. In such cases, it is impractical to enumerate all the external references in every source file. This allows the assembler and linker to do the heavy lifting while not preventing a particular source module from defining a local symbol of the same name as an external symbol if it does not need the external symbol. (This pragma will not cause an automatic external definition if there is already a locally defined symbol.)

  This pragma will often be specified on the command line for large projects. However, depending on the specific dynamics of the project, it may be sufficient for one or two files to use this pragma internally.

export

  This pragma causes all symbols to be added to the export list automatically. This is useful when a large number of symbols need to be exported but you do not wish to include an EXPORT directive for all of them. This is often useful on the command line but might be useful even inline with the PRAGMA directive if a large number of symbols in a row are to be exported.

dollarlocal

  When set, a "$" in a symbol makes it local. When not set, "$" does not cause a symbol to be local. It is set by default except when using the OS9 target.

dollarnotlocal

  This is the same as the "dollarlocal" pragma except its sense is reversed. That is, "dollarlocal" and "nodollarnotlocal" are equivalent and "nodollarlocal" and "dollarnotlocal" are equivalent.

pcaspcr

  Normally, LWASM makes a distinction between PC and PCR in program counter relative addressing. In particular, the use of PC means an absolute offset from PC while PCR causes the assembler to calculate the offset to the specified operand and use that as the offset from PC. By setting this pragma, you can have PC treated the same as PCR.

shadow

  When this pragma is in effect, it becomes possible to define a macro that matches an internal operation code. Thus, it makes it possible to redefine either CPU instructions or pseudo operations. Because this feature is of dubious utility, it is disabled by default.

nolist

  Lines where this pragma is in effect will not appear in the assembly listing. Also, any symbols defined under this pragma will not show up in the symbol list. This is most useful in include files to avoid spamming the assembly listing with dozens, hundreds, or thousands of irrelevant symbols.

autobranchlength

  One of the perennial annoyances for 6809 programmers is that the mneumonics for the short and long branch instructions are different (bxx vs. lbxx), which is at odds with the rest of the instruction set. This pragma is a solution to those annoying byte overflow errors that short branch instructions tend to aquire.

  When this pragma is in effect, which is not the default, whenever any relative branch instruction is used, its size will be automatically determined based on the actual distance to the destination. In other words, one can write code with long or short branches everywhere and the assembler will choose a size for the branch.

  Also, while this pragma is in effect, the > and < symbols can be used to force the branch size, analogous to their use for other instructions with < forcing 8 bit offsets and > forcing 16 bit offets.

  Because this pragma leads to source that is incompatible with other assemblers, it is strongly recommended that it be invoked using the PRAGMA directive within the source code rather than on the command line or via the *PRAGMA directive. This way, an error will be raised if someone tries to assemble the code under a different assembler.

  Note that if the "forwardrefmax" pragma is ineffect, as is the current default, this pragma will not behave as expected.

nosymbolcase, symbolnocase

  Any symbol defined while this pragma is in force will be treated as case insensitive, regardless whether the pragma is in force when the symbol is referenced.

  It is important to note that this pragma will not work as expected in all cases when using the object file assembly target. It is intended for use only when the assembler will be producing the final binary.

condundefzero

  This pragma will cause the assembler to change the way it handles symbols in conditional expressions. Ordinarily, any symbol that is not defined prior to the conditional will throw an undefined symbol error. With this pragma in effect, symbols that are not yet defined at the point the conditional is encountered will be treated as zero.

  This is not the default because it encourages poor code design. One should use the "IFDEF" or "IFNDEF" conditionals to test for the presence of a symbol.

  It is important to note that if a symbol is defined but it does not yet evaluate to a constant value at the point where the conditional appears, the assembler will still complain about a non constant condition.

forwardrefmax

  This pragma will disable forward reference optimization completely. However, many source files, especially those not using the PCR relative addressing modes, this optimization is pointless since the assembler will almost certainly settle on a 16 bit offset or address. If all variables in the direct page are defined before the main body of the code, the benefit of forward reference optimization almost certainly vanishes completely. However, the cost of doing that optimization remains and can result in a very long assembly time. Because of this, "forwardrefmax" has been the default setting since version 4.14 of LWTOOLS. To turn it off, use "noforwardrefmax".

  Enabling this pragma will cause all forward references to use the maximum offset or address size, much the same has EDTASM and other pure two pass assemblers do. The side effect is that all line lengths and symbol values are fully resolved after the initial parsing pass and the amount of work to resolve everything becomes almost nil.

  While this pragma can be applied selectively to sections of source code (use *PRAGMA if doing so and compatibility with other assemblers is desired), it is likely more useful when provided as a command line pragma.

  It should be noted that the presence or absence of this pragma will not change the correctness of the generated code unless cycle counts or byte counts are critical (which they usually are not). It also will not override the operand size override prefixes (< and >). It only applies when the assembler is left to guess what the operand size is.

operandsizewarning

  Enabling this pragma will cause LWASM to show a warning when it detects that a smaller addressing mode could be used for an instruction. This is particularly useful for finding places where long branches are used where short branches could be used instead. It will also show the warnings for indexing offsets (regardless of whether the operand size is forced).

  As of LWASM 4.16, no other checks are performed.

qrts

  Enables the use of the ?RTS branch target. ?RTS is implemented to maintain compatibility with the MACRO-80c assembler. It works by searching backward in the code for an RTS instruction. If none is found, it inverts the branch logic and inserts an RTS following the branch instruction. Below you can see how a BMI (2B xx) has been assembled as a BPL *+1 (2A 01) to skip over an inserted RTS (39).

      1D1E 7D1D1D            TST   WHICH1
      1D21 2A0139            BMI   ?RTS
      1D24 BD1D65            JSR   INV

m80ext

  This pragma (along with pragma qrts) enables some uncommon behaviors to accomodate The Micro Works MACRO-80c assembler from 1982. This assembler was used by a number of notable TRS-80 Color Computer applications and the goal of this pragma is to allow them to build identical binaries from unmodified, vintage source code.

  In m80ext mode, the handling of the "END" pseudo-op changes when used inside an include file. Instead of terminating all assembly, it merely stops processing of the current include file (this behavior matches the original Motorola 6809 assembler). In addition, loading an ASCII value with a single quote (e.g., LDA #'N) is extended to 16-bit registers (e.g., LDD #'NO). LWASM normally supports this via double quote and that is the proper use in modern code. Finally, the FCC pseudo-op is extended to handle FCB-like behavior after the closing delimiter:

                             FCC "Greetings from 1982",13,0

testmode

  This pragma is intended for internal testing purposes. In testmode, the assembler searches for a specially-formatted comment starting with a semicolon followed by a period. Immediately afterward are a list of hex bytes that the assembler is expected to generate. Likewise, if the assembler is expected to throw an error or warning on a given line, you can check by specifying "E:" followed by the error number. In this case the error is ignored and the assembler continues ignoring the line in question.

      1D1E 7D1D1D            TST   WHICH1    ;.7d1d1d
      1D21 2A0139            BMI   ?RTS      ;.2a0139
      1D24 1D24              FDB   *         ;.1d24
      1D26                   xyz   INV       ;.E:32    (Error 32 is "Bad opcode")

emuext

  This pragma enables two instructions useful when running code in compatible emulators. Break breaks into the debugger. Log writes printf-style output to the debug window

            LOG           ; log output
            FDB   FSTR    ; pointer to format string
            FDB   PX1     ; 16 bit pointer to 16 bit value
            FDB   PY1     ; 16 bit pointer to 8 bit value (see format string!)
            FDB   PX2     ; 16 bit pointer to 16 bit value
            FDB   PY2     ; 16 bit pointer to 8 bit value
            ; execution continues here ...
            RTS

      ; format string
      FSTR  FCC   "%hu,%hhu - %hu,%hhu"
            FCB   10,0

As a convenience, each input file has a pragma state stack. This allows, through the use of *PRAGMAPUSH and *PRAGMAPOP, a file to change a pragma state and then restore it to the precise state it had previously. If, at the end of an input file, all pragma states have not been popped, they will be removed from the stack. Thus, it is critical to employ *PRAGMAPOP correctly. Because each input file has its own pragma stack, using *PRAGMAPUSH in one file and *PRAGMAPOP in another file will not work.

Pragma stacks are more useful in include files, in particular in conjunction with the nolist pragma. One can push the state of the nolist pragma, engage the nolist pragma, and then pop the state of the nolist pragma at the end of the include file. This will cause the entire include file to operate under the nolist pragma. However, if the file is included while nolist is already engaged, it will not undo that state.

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