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26 Interface

26.1 Introduction

This chapter is under construction!

This chapter describes some of the internals of vasm and tries to explain what has to be done to write a cpu module, a syntax module or an output module for vasm. However if someone wants to write one, I suggest to contact me first, so that it can be integrated into the source tree.

Note that this documentation may mention explicit values when introducing symbolic constants. This is due to copying and pasting from the source code. These values may not be up to date and in some cases can be overridden. Therefore do never use the absolute values but rather the symbolic representations.

26.2 Building vasm

This section deals with the steps necessary to build the typical vasm executable from the sources.

26.2.1 Directory Structure

The vasm-directory contains the following important files and directories:


The main directory containing the assembler sources.


The Makefile used to build vasm.


Directories for the syntax modules.


Directories for the cpu modules.


Directory the object modules will be stored in.

All compiling is done from the main directory and the executables will be placed there as well. The main assembler for a combination of <cpu> and <syntax> will be called vasm<cpu>_<syntax>. All output modules are usually integrated in every executable and can be selected at runtime.

26.2.2 Adapting the Makefile

Before building anything you have to insert correct values for your compiler and operating system in the ‘Makefile’.


Here you may define an extension which is appended to the executable’s name. Useful, if you build various targets in the same directory.


Defines the file name extension for executable files. Not needed for most operating systems. For Windows it would be ‘.exe’.


Here you have to insert a command that invokes an ANSI C compiler you want to use to build vasm. It must support the ‘-I’ option the same like e.g. vc or gcc.


Here you will usually define an option like ‘-c’ to instruct the compiler to generate an object file. Additional options, like the optimization level, should also be inserted here as well. When the host operating system is different from a Unix (MacOSX and MiNT are Unix), you have to define one of the following preprocessor macros:


AmigaOS (M68k or PPC), MorphOS, AROS.


Atari TOS.


CP/M, MS-DOS, Windows.


Here you define the option which is used to specify the name of an output file, which is usually ‘-o’.


Here you insert a command which starts the linker. This may be the the same as under CC.


Here you have to add options which are necessary for linking. E.g. some compilers need special libraries for floating-point.


Here you define the option which is used by the linker to specify the output file name.


Specify a command to delete a file, e.g. rm -f.

An example for the Amiga using vbcc would be:

      TARGET = _os3
      CC = vc +aos68k
      CCOUT = -o
      COPTS = -c -c99 -cpu=68020 -DAMIGA -O1
      LD = $(CC)
      LDOUT = $(CCOUT)
      LDFLAGS = -lmieee
      RM = delete force quiet

An example for a typical Unix-installation would be:

      TARGET =
      CC = gcc
      CCOUT = -o
      COPTS = -c -O2
      LD = $(CC)
      LDOUT = $(CCOUT)
      LDFLAGS = -lm
      RM = rm -f

Open/Net/Free/Any BSD i386 systems will probably require the following an additional ‘-D_ANSI_SOURCE’ in COPTS.

26.2.3 Building vasm

Note to users of Open/Free/Any BSD i386 systems: You will probably have to use GNU make instead of BSD make, i.e. in the following examples replace "make" with "gmake".


      make CPU=<cpu> SYNTAX=<syntax>

For example:

      make CPU=ppc SYNTAX=std

The following CPU modules can be selected:

The following syntax modules can be selected:

For Windows and various Amiga targets there are already Makefiles included, which you may either copy on top of the default ‘Makefile’, or call it explicitely with make’s ‘-f’ option:

    make -f Makefile.OS4 CPU=ppc SYNTAX=std

26.3 General data structures

This section describes the fundamental data structures used in vasm which are usually necessary to understand for writing any kind of module (cpu, syntax or output). More detailed information is given in the respective sections on writing specific modules where necessary.

26.3.1 Source

A source structure represents a source text module, which can be either the main source text, an included file or a macro. There is always a link to the parent source from where the current source context was included or called.

struct source *parent;

Pointer to the parent source context. Assembly continues there when the current source context ends.

int parent_line;

Line number in the parent source context, from where we were called. This information is needed, because line numbers are only reliable during parsing and later from the atoms. But an include directive doesn’t create an atom.

struct source_file *srcfile;

The source_file structure has the unique file name, index and text-pointer for this source text instance. Used for debugging output, like DWARF.

char *name;

File name of the main source or include file, or macro name.

char *text;

Pointer to the source text start.

size_t size;

Size of the source text to assemble in bytes.

struct source *defsrc;

This is a NULL-pointer for real source text files. Otherwise it is a reference to the source which defines the current macro or repetition.

int defline;

Valid when defsrc is not NULL. Contains the starting line number of a macro or repetition in a source text file.

macro *macro;

Pointer to macro structure, when currently inside a macro (see also num_params).

unsigned long repeat;

Number of repetitions of this source text. Usually this is 1, but for text blocks between a rept and endr directive it allows any number of repetitions, which is decremented everytime the end of this source text block is reached.

char *irpname;

Name of the iterator symbol in special repeat loops which use a sequence of arbitrary values, being assigned to this symbol within the loop. Example: irp directive in std-syntax.

struct macarg *irpvals;

A list of arbitrary values to iterate over in a loop. With each iteration the frontmost value is removed from the list until it is empty.

int cond_level;

Current level of conditional nesting while entering this source text. It is automatically restored to the previous level when leaving the source prematurely through end_source().

struct macarg *argnames;

The current list of named macro arguments.

int num_params;

Number of macro parameters passed at the invocation point from the parent source. For normal source files this entry will be -1. For macros 0 (no parameters) or higher.

char *param[MAXMACPARAMS];

Pointer to the macro parameters.

int param_len[MAXMACPARAMS];

Number of characters per macro parameter.

int num_quals;

(If MAX_QUALIFIERS!=0.) Number of qualifiers for a macro. when not passed on invocation these are the default qualifiers.

char *qual[MAX_QUALIFIERS];

(If MAX_QUALIFIERS!=0.) Pointer to macro qualifiers.

int qual_len[MAX_QUALIFIERS];

(If MAX_QUALIFIERS!=0.) Number of characters per macro qualifier.

unsigned long id;

Every source has its unique id. Useful for macros supporting the special \@ parameter.

char *srcptr;

The current source text pointer, pointing to the beginning of the next line to assemble.

int line;

Line number in the current source context. After parsing the line number of the current atom is stored here.

size_t bufsize;

Current size of the line buffer (linebuf). The size of the line buffer is extended automatically, when an overflow happens.

char *linebuf;

A buffer for the current line being assembled in this source text. A child-source, like a macro, can refer to arguments from this buffer, so every source has got its own. When returning to the parent source, the linebuf is deallocated to save memory.

expr *cargexp;

(If CARGSYM was defined.) Pointer to the current expression assigned to the CARG-symbol (used to select a macro argument) in this source instance. So it can be restored when reentering this instance.

long reptn;

(If REPTNSYM was defined.) Current value of the repetition counter symbol in this source instance. So it can be restored when reentering this instance.

26.3.2 Sections

One of the top level structures is a linked list of sections describing continuous blocks of memory. A section is specified by an object of type section with the following members that can be accessed by the modules:

struct section *next;

A pointer to the next section in the list.

char *name;

The name of the section.

char *attr;

A string describing the section flags in ELF notation (see, for example, documentation o the .section directive of the standard syntax mopdule.

atom *first;
atom *last;

Pointers to the first and last atom of the section. See following sections for information on atoms.

taddr align;

Alignment of the section in bytes.

uint32_t flags;

Flags of the section. Currently available flags are:


At least one symbol is defined in this section.


The current atom changed its size multiple times, so atom_size() is now called with this flag set in its section to make the backend (e.g. instruction_size()) aware of it and do less aggressive optimizations.


Section is unallocated, which means it doesn’t use any memory space in the output file. Such a section will be removed before creating the output file and all its labels converted into absolute expression symbols. Used for "offset" sections. Refer to switch_offset_section().


As long as this flag is set new labels in a section are defined as local labels, with the section name as global parent label.


Section is loaded at an absolute address in memory.


Remembers state of the ABSOLUTE flag before entering relocated-org mode (IN_RORG). So it can be restored later.


Section has entered relocated-org mode, which also sets the ABSOLUTE flag. In this mode code is written into the current section, but relocated to an absolute address. No relocation information are generated.


Section is marked as suitable for cpu-specific "near" addressing modes. For example, base-register relative. The cpu backend can use this information as an optimization hint when referencing symbols from this section.

taddr org;

Start address of a section. Usually zero.

taddr pc;

Current address in this section. Can be used while traversing through the section. Has to be updated by a module using it. Is set to org at the beginning.

unsigned long idx;

A member usable by the output module for private purposes.

26.3.3 Symbols

Symbols are represented by a linked list of type symbol with the following members that can be accessed by the modules:.

int type;

Type of the symbol. Available are:

#define LABSYM 1

The symbol is a label defined at a specific location.

#define IMPORT 2

The symbol is imported from another file.

#define EXPRESSION 3

The symbol is defined using an expression.

uint32_t flags;

Flags of this symbol. Available are:

#define TYPE_UNKNOWN 0

The symbol has no type information.

#define TYPE_OBJECT 1

The symbol defines an object.


The symbol defines a function.

#define TYPE_SECTION 3

The symbol defines a section.

#define TYPE_FILE 4

The symbol defines a file.

#define EXPORT (1<<3)

The symbol is exported to other files.

#define INEVAL (1<<4)

Used internally.

#define COMMON (1<<5)

The symbol is a common symbol.

#define WEAK (1<<6)

The symbol is weak, which means the linker may overwrite it with any global definition of the same name. Weak symbols may also stay undefined, in which case the linker would assign them a value of zero.

#define LOCAL (1<<7)

Only informational. A symbol can be explicitely declared as local by a syntax-module directive.

#define VASMINTERN (1<<8)

Vasm-internal symbol, which is usually not exported into an output file.

#define PROTECTED (1<<9)

Used internally to protect the current-PC symbol from deletion.

#define REFERENCED (1<<10)

Symbol was referenced in the source and a relocation entry has been created.

#define ABSLABEL (1<<11)

Label was defined inside an absolute section, or during relocated-org mode. So it has an absolute address and will not generate a relocation entry when being referenced.

#define EQUATE (1<<12)

Symbols flagged as EQUATE are constant and its value must not be changed.

#define REGLIST (1<<13)

Symbol is a register list definition.

#define USED (1<<14)

Symbol appeared in an expression. Symbols which were only defined, (as label or equte) and never used throughout the whole source, don’t get this flag set.

#define NEAR (1<<15)

Symbol may be referenced by "near" addressing mode. For example, base register relative. Used as an optimization hint in the cpu backend.

#define RSRVD_S (1L<<24)

The range from bit 24 to 27 (counted from the LSB) is reserved for use by the syntax module.

#define RSRVD_O (1L<<28)

The range from bit 28 to 31 (counted from the LSB) is reserved for use by the output module.

The type-flags can be extracted using the TYPE() macro which expects a pointer to a symbol as argument.

char *name;

The name of the symbol.

expr *expr;

The expression in case of EXPRESSION symbols.

expr *size;

The size of the symbol, if specified.

section *sec;

The section a LABSYM symbol is defined in.

taddr pc;

The address of a LABSYM symbol.

taddr align;

The alignment of the symbol in bytes.

unsigned long idx;

A member usable by the output module for private purposes.

26.3.4 Register symbols

Optional register symbols are available when the backend defines HAVE_REGSYMS in ‘cpu.h’ together with the hash table size. Example:

#define REGSYMHTSIZE 256

A register symbol is defined by an object of type regsym with the following members that can be accessed by the modules:

char *reg_name;

Symbol name.

int reg_type;

Optional type of register.

unsigned int reg_flags;

Optional register symbol flags.

unsigned int reg_num;

Register number or value.

Refer to ‘symbol.h’ for functions to create and find register symbols.

26.3.5 Atoms

The contents of each section are a linked list built out of non-separable atoms. The general structure of an atom is:

typedef struct atom {
  struct atom *next;
  int type;
  taddr align;
  taddr lastsize;
  unsigned changes;
  source *src;
  int line;
  listing *list;
  union {
    instruction *inst;
    dblock *db;
    symbol *label;
    sblock *sb;
    defblock *defb;
    void *opts;
    int srcline;
    char *ptext;
    printexpr *pexpr;
    expr *roffs;
    taddr *rorg;
    assertion *assert;
    aoutnlist *nlist;
  } content;
} atom;

The members have the following meaning:

struct atom *next;

Pointer to the following atom (0 if last).

int type;

The type of the atom. Can be one of

#define VASMDEBUG 0

Used for internal debugging.

#define LABEL 1

A label is defined here.

#define DATA 2

Some data bytes of fixed length and constant data are put here.


Generally refers to a machine instruction or pseudo/opcode. These atoms can change length during optimization passes and will be translated to DATA-atoms later.

#define SPACE 4

Defines a block of data filled with one value (byte). BSS sections usually contain only such atoms, but they are also sometimes useful as shorter versions of DATA-atoms in other sections.

#define DATADEF 5

Defines data of fixed size which can contain cpu specific operands and expressions. Will be translated to DATA-atoms later.

#define LINE 6

A source text line number (usually from a high level language) is bound to the atom’s address. Useful for source level debugging in certain ABIs.

#define OPTS 7

A means to change assembler options at a specific source text line. For example optimization settings, or the cpu type to generate code for. The cpu module has to define HAVE_CPU_OPTS and export the required functions if it wants to use this type of atom.

#define PRINTTEXT 8

A string is printed to stdout during the final assembler pass. A newline is automatically appended.

#define PRINTEXPR 9

Prints the value of an expression during the final assembler pass to stdout.

#define ROFFS 10

Set the program counter to an address relative to the section’s start address. These atoms will be translated into SPACE atoms in the final pass.

#define RORG 11

Assemble this block under the given base address, while the code is still written into the original memory region.

#define RORGEND 12

Ends a RORG block and returns to the original addessing.

#define ASSERT 13

The assertion expression is checked in the final pass and an error message is generated (using the expression string and an optional message out of this atom) when it evaluates to 0.

#define NLIST 14

Defines a stab-entry for the a.out object file format. nlist-style stabs can also occur embedded in other object file formats, like ELF.

taddr align;

The alignment of this atom. Address must be dividable by align.

taddr lastsize;

The size of this atom in the last resolver pass. When the size has changed in the current pass, the assembler will request another resolver run through the section.

unsigned changes;

Number of changes in the size of this atom since pass number FASTOPTPHASE. An increasing number usually indicates a problem in the cpu backend’s optimizer and will be flagged by setting RESOLVE_WARN in the Section flags, as soon as changes exceeds MAXSIZECHANGES. So the backend can choose not to optimize this atom as aggressive as before.

source *src;

Pointer to the source text object to which this atom belongs.

int line;

The source line number that created this atom.

listing *list;

Pointer to the listing object to which this atoms belong.

instruction *inst;

(In union content.) Pointer to an instruction structure in the case of an INSTRUCTION-atom. Contains the following elements:

int code;

The cpu specific code of this instruction.

char *qualifiers[MAX_QUALIFIERS];

(If MAX_QUALIFIERS!=0.) Pointer to the qualifiers of this instruction.

operand *op[MAX_OPERANDS];

(If MAX_OPERANDS!=0.) The cpu-specific operands of this instruction.

instruction_ext ext;

(If the cpu module defines HAVE_INSTRUCTION_EXTENSION.) A cpu-module-specific structure. Typically used to store appropriate opcodes, allowed addressing modes, supported cpu derivates etc.

dblock *db;

(In union content.) Pointer to a dblock structure in the case of a DATA-atom. Contains the following elements:

taddr size;

The number of bytes stored in this atom.

char *data;

A pointer to the data.

rlist *relocs;

A pointer to relocation information for the data.

symbol *label;

(In union content.) Pointer to a symbol structure in the case of a LABEL-atom.

sblock *sb;

(In union content.) Pointer to a sblock structure in the case of a SPACE-atom. Contains the following elements:

size_t space;

The size of the empty/filled space in bytes.

expr *space_exp;

The above size as an expression, which will be evaluated during assembly and copied to space in the final pass.

size_t size;

The size of each space-element and of the fill-pattern in bytes.

unsigned char fill[MAXBYTES];

The fill pattern, up to MAXBYTES bytes.

expr *fill_exp;

Optional. Evaluated and copied to fill in the final pass, when not null.

rlist *relocs;

A pointer to relocation information for the space.

taddr maxalignbytes;

An optional number of maximum padding bytes to fulfil the atom’s alignment requirement. Zero means there is no restriction.

uint32_t flags;

The output module should not allocate any file space for this atom, when possible (example: DataBss section, as supported by the "hunkexe" output file format). It is not needed to set this flag when the output section is BSS.

defblock *defb;

(In union content.) Pointer to a defblock structure in the case of a DATADEF-atom. Contains the following elements:

taddr bitsize;

The size of the definition in bits.

operand *op;

Pointer to a cpu-specific operand structure.

void *opts;

(In union content.) Points to a cpu module specific options object in the case of a OPTS-atom.

int srcline;

(In union content.) Line number for source level debugging in the case of a LINE-atom.

char *ptext;

(In union content.) A string to print to stdout in case of a PRINTTEXT-atom.

printexpr *pexpr;

(In union content.) Pointer to a printexpr structure in the case of a PRINTEXPR-atom. Contains the following elements:

expr *print_exp;

Pointer to an expression to evaluate and print.

short type;

Format type of the printed value. We can print as hexadecimal (PEXP_HEX), signed decimal (PEXP_SDEC), unsigned decimal (PEXP_UDEC), binary (PEXP_BIN) OR ASCII (PEXP_ASC).

short size;

Size (precision) of the printed value in bits. Excessive bits will be masked out, and sign-extended when requested.

expr *roffs;

(In union content.) The expression holds the relative section offset to align to in case of a ROFFS-atom.

taddr *rorg;

(In union content.) Assemble the code under the base address in rorg in case of a RORG-atom.

assertion *assert;

(In union content.) Pointer to an assertion structure in the case of an ASSERT-atom. Contains the following elements:

expr *assert_exp;

Pointer to an expression which should evaluate to non-zero.

char *exprstr;

Pointer to the expression as text (to be used in the output).

char *msgstr;

Pointer to the message, which would be printed when assert_exp evaluates to zero.

aoutnlist *nlist;

(In union content.) Pointer to an nlist structure, describing an aout stab entry, in case of an NLIST-atom. Contains the following elements:

char *name;

Name of the stab symbol.

int type;

Symbol type. Refer to stabs.h for definitions.

int other;

Defines the nature of the symbol (function, object, etc.).

int desc;

Debugger information.

expr *value;

Symbol’s value.

26.3.6 Relocations

DATA and SPACE atoms can have a relocation list attached that describes how this data must be modified when linking/relocating. They always refer to the data in this atom only.

There are a number of predefined standard relocations and it is possible to add other cpu-specific relocations. Note however, that it is always preferrable to use standard relocations, if possible. Chances that an output module supports a certain relocation are much higher if it is a standard relocation.

A relocation list uses this structure:

typedef struct rlist {
  struct rlist *next;
  void *reloc;
  int type;
} rlist;

Type identifies the relocation type. All the standard relocations have type numbers between FIRST_STANDARD_RELOC and LAST_STANDARD_RELOC. Consider ‘reloc.h’ to see which standard relocations are available.

The detailed information can be accessed via the pointer reloc. It will point to a structure that depends on the relocation type, so a module must only use it if it knows the relocation type.

All standard relocations point to a type nreloc with the following members:

size_t byteoffset;

Offset in bytes, from the start of the current DATA atom, to the beginning of the relocation field. This may also be the address which is used as a basis for PC-relative relocations. Or a common basis for several separated relocation fields, which will be translated into a single relocation type by the output module.

size_t bitoffset;

Offset in bits to the beginning of the relocation field, adds to byteoffset*bitsperbyte. Bits are counted in a bit-stream from lower to higher address bytes. But note, that inside a little-endian byte they are counted from the LSB to the MSB, while they are counted from the MSB to the LSB for big-endian targets.

int size;

The size of the relocation field in bits.

taddr mask;

The mask defines which portion of the relocated value is set by this relocation field.

taddr addend;

Value to be added to the symbol value.

symbol *sym;

The symbol referred by this relocation

To describe the meaning of these entries, we will define the steps that shall be executed when performing a relocation:

  1. Extract the size bits from the data atom, starting with bit number byteoffset*bitsperbyte+bitoffset. We start counting bits from the lowest to the highest numbered byte in memory. Inside a big-endian byte we count from the MSB to the LSB. Inside a little-endian byte we count from the LSB to the MSB.
  2. Determine the relocation value of the symbol. For a simple absolute relocation, this will be the value of the symbol sym plus the addend. For other relocation types, more complex calculations will be needed. For example, in a program-counter relative relocation, the value will be obtained by subtracting the address of the data atom plus byteoffset from the value of sym plus addend.
  3. Calculate the bit-wise "and" of the value obtained in the step above and the mask value.
  4. Normalize, i.e. shift the value above right as many bit positions as there are low order zero bits in mask.
  5. Add this value to the value extracted in step 1.
  6. Insert the low order size bits of this value into the data atom starting with bit byteoffset*bitsperbyte+bitoffset.

26.3.7 Errors

Each module can provide a list of possible error messages contained e.g. in ‘syntax_errors.h’ or ‘cpu_errors.h’. They are a comma-separated list of a printf-format string and error flags. Allowed flags are WARNING, ERROR, FATAL, MESSAGE and NOLINE. They can be combined using or (|). NOLINE has to be set for error messages during initialiation or while writing the output, when no source text is available. Errors cause the assembler to return false. FATAL causes the assembler to terminate immediately.

The errors can be emitted using the function syntax_error(int n,...), cpu_error(int n,...) or output_error(int n,...). The first argument is the number of the error message (starting from zero). Additional arguments must be passed according to the format string of the corresponding error message.

26.4 Syntax modules

A new syntax module must have its own subdirectory under ‘vasm/syntax’. At least the files ‘syntax.h’, ‘syntax.c’ and ‘syntax_errors.h’ must be written.

26.4.1 The file ‘syntax.h


These macros should return non-zero if and only if the argument is a valid character to start an identifier or a valid character inside an identifier, respectively. ISIDCHAR must be a superset of ISIDSTART.

#define ISBADID(p,l)

Even with ISIDSTART and ISIDCHAR checked, there may be combinations of characters which do not form a valid initializer (for example, a single character). This macro returns non-zero, when this is the case. First argument is a pointer to the new identifier and second is its length.

#define ISEOL(x)

This macro returns true when the string pointing at x is either a comment character or end-of-line.

#define CHKIDEND(s,e) chkidend((s),(e))

Defines an optional function to be called at the end of the identifier recognition process. It allows you to adjust the length of the identifier by returning a modified e. Default is to return e. The function is defined as char *chkidend(char *startpos,char *endpos).

#define BOOLEAN(x) -(x)

Defines the result of boolean operations. Usually this is (x), as in C, or -(x) to return -1 for True.

#define NARGSYM "NARG"

Defines the name of an optional symbol which contains the number of arguments in a macro.

#define CARGSYM "CARG"

Defines the name of an optional symbol which can be used to select a specific macro argument with \., \+ and \-.


Defines the name of an optional symbol containing the counter of the current repeat iteration.

#define EXPSKIP() s=exp_skip(s)

Defines an optional replacement for skip() to be used in expr.c, to skip blanks in an expression. Useful to forbid blanks in an expression and to ignore the rest of the line (e.g. to treat the rest as comment). The function is defined as char *exp_skip(char *stream).


Should be defined when the syntax module wants to ignore the operand field on instructions without an operand. Useful, when everything following an operand should be regarded as comment, without a comment character.


Optionally defines the maximum number of macro arguments, if you need more than the default number of 9.

#define SKIP_MACRO_ARGNAME(p) skip_identifier(p)

An optional function to skip a named macro argument in the macro definition. Argument is the current source stream pointer. The default is to skip an identifier.

#define MACRO_ARG_OPTS(m,n,a,p) NULL

An optional function to parse and skip options, default values and qualifiers for each macro argument. Returns NULL when no argument options have been found. Arguments are:

struct macro *m;

Pointer to the macro structure being currently defined.

int n;

Argument index, starting with zero.

char *a;

Name of this argument.

char *p;

Current source stream pointer. An updated pointer will be returned.

Defaults to unused.

#define MACRO_ARG_SEP(p) (*p==',' ? skip(p+1) : NULL)

An optional function to skip a separator between the macro argument names in the macro definition. Returns NULL when no valid separator is found. Argument is the current source stream pointer. Defaults to using comma as the only valid separator.

#define MACRO_PARAM_SEP(p) (*p==',' ? skip(p+1) : NULL)

An optional function to skip a separator between the macro parameters in a macro call. Returns NULL when no valid separator is found. Argument is the current source stream pointer. Defaults to using comma as the only valid separator.

#define EXEC_MACRO(s)

An optional function to be called just before a macro starts execution. Parameters and qualifiers are already parsed. Argument is the source pointer of the new macro. Defaults to unused.

26.4.2 The file ‘syntax.c

A syntax module has to provide the following elements (all other funtions should be static to prevent name clashes):

char *syntax_copyright;

A string that will be emitted as part of the copyright message.

hashtable *dirhash;

A pointer to the hash table with all directives.

char commentchar;

A character used to introduce a comment until the end of the line.

char *defsectname;

Name of a default section which vasm creates when a label or code occurs in the source, but the programmer forgot to specify a section. Assigning NULL means that there is no default and vasm will show an error in this case.

char *defsecttype;

Type of the default section (see above). May be NULL.

int init_syntax();

Will be called during startup, after argument parsing Must return zero if initializations failed, non-zero otherwise.

int syntax_args(char *);

This function will be called with the command line arguments (unless they were already recognized by other modules). If an argument was recognized, return non-zero.

char *skip(char *);

A function to skip whitespace etc.

char *skip_operand(char *);

A function to skip an instruction’s operand. Will terminate at end of line or the next comma, returning a pointer to the rest of the line behind the comma.

void eol(char *);

This function should check that the argument points to the end of a line (only comments or whitespace following). If not, an error or warning message should be omitted.

char *const_prefix(char *,int *);

Check if the first argument points to the start of a constant. If yes return a pointer to the real start of the number (i.e. skip a prefix that may indicate the base) and write the base of the number through the pointer passed as second argument. Return zero if it does not point to a number.

char *const_suffix(char *,char *);

First argument points to the start of the constant (including prefix) and the second argument to first character after the constant (excluding suffix). Checks for a constant-suffix and skips it. Return pointer to the first character after that constant. Example: constants with a ’h’ suffix to indicate a hexadecimal base.

void parse(void);

This is the main parsing function. It has to read lines via the read_next_line() function, parse them and create sections, atoms and symbols. Pseudo directives are usually handled by the syntax module. Instructions can be parsed by the cpu module using parse_instruction().

char *parse_macro_arg(struct macro *,char *,struct namelen *,struct namelen *);

Called to parse a macro parameter by using the source stream pointer in the second argument. The start pointer and length of a single passed parameter is written to the first struct namelen, while the optionally selected named macro argument is passed in the second struct namelen. When the len field of the second namelen is zero, then the argument is selected by position instead by name. Returns the updated source stream pointer after successful parsing.

int expand_macro(source *,char **,char *,int);

Expand parameters and special commands inside a macro source. The second argument is a pointer to the current source stream pointer, which is updated on any succesful expansion. The function will return the number of characters written to the destination buffer (third argument) in this case. Returning -1 means: no expansion took place. The last argument defines the space in characters which is left in the destination buffer.

char *get_local_label(char **);

Gets a pointer to the current source pointer. Has to check if a valid local label is found at this point. If yes return a pointer to the vasm-internal symbol name representing the local label and update the current source pointer to point behind the label.

Have a look at the support functions provided by the frontend to help.

26.5 CPU modules

A new cpu module must have its own subdirectory under ‘vasm/cpus’. At least the files ‘cpu.h’, ‘cpu.c’ and ‘cpu_errors.h’ must be written.

26.5.1 The file ‘cpu.h

A cpu module has to provide the following elements (all other functions should be static to prevent name clashes) in cpu.h:

#define MAX_OPERANDS 3

Maximum number of operands of one instruction.


Maximum number of mnemonic-qualifiers per mnemonic.


Define this, when qualifiers shouldn’t be allowed for macros. For some architectures, like ARM, macro qualifiers make no sense.

typedef int32_t taddr;

Data type to represent a target-address. Preferrably use the ones from ‘stdint.h’.

typedef uint32_t utaddr;

Unsigned data type to represent a target-address.

#define BIGENDIAN 0

Define these according to the target endianess. For CPUs which support big- and little-endian, you may assign a global variable here. So be aware of it, and never use #if BIGENDIAN, but always if(BIGENDIAN) in your code.

#define VASM_CPU_<cpu> 1

Insert the cpu specifier.

#define INST_ALIGN 2

Minimum instruction alignment.

#define DATA_ALIGN(n) ...

Default alignment for n-bit data. Can also be a function.

#define DATA_OPERAND(n) ...

Operand class for n-bit data definitions. Can also be a function. Negative values denote a floating point data definition of -n bits.

typedef ... operand;

Structure to store an operand.

typedef ... mnemonic_extension;

Mnemonic extension.

Optional features, which can be enabled by defining the following macros:


If cpu-specific data should be added to all instruction atoms.

typedef ... instruction_ext;

Type for the above extension.


Backend requires a zeroed operand structure when calling parse_operand() for the first time. Defaults to undefined.


Valid opening parenthesis for instruction operands. Defaults to '('.


Valid closing parenthesis for instruction operands. Defaults to ')'.


An optional function with the arguments (int idx). Returns true when the mnemonic with index idx is valid for the current state of the backend (e.g. it is available for the selected cpu architecture).

#define MNEMOHTABSIZE 0x4000

You can optionally overwrite the default hash table size defined in ‘vasm.h’. May be necessary for larger mnemonic tables.


When defined, this is a function with the arguments (operand *op,int type), which returns true when the given operand type (type) is optional. The function is only called for missing operands and should also initialize op with default values (e.g. 0).

Implementing additional target-specific unary operations is done by defining the following optional macros:

#define EXT_UNARY_NAME(s)

Should return True when the string in s points to an operation name we want to handle.

#define EXT_UNARY_TYPE(s)

Returns the operation type code for the string in s. Note that the last valid standard operation is defined as LAST_EXP_TYPE, so the target-specific types will start with LAST_EXP_TYPE+1.

#define EXT_UNARY_EVAL(t,v,r,c)

Defines a function with the arguments (int t, taddr v, taddr *r, int c) to handle the operation type t returning an int to indicate whether this type has been handled or not. Your operation will by applied on the value v and the result is stored in *r. The flag c is passed as 1 when the value is constant (no relocatable addresses involved).

#define EXT_FIND_BASE(b,e,s,p)

Defines a function with the arguments (symbol **b, expr *e, section *s, taddr p) to save a pointer to the base symbol of expression e into the symbol pointer, pointed to by b. The type of this base is given by an int return code. Further on, e->type has to checked to be one of the operations to handle. The section pointer s and the current pc p are needed to call the standard find_base() function.

26.5.2 The file ‘cpu.c

A cpu module has to provide the following elements (all other functions and data should be static to prevent name clashes) in cpu.c:

int bitsperbyte;

The number of bits per byte of the target cpu.

int bytespertaddr;

The number of bytes per taddr.

mnemonic mnemonics[];

The mnemonic table keeps a list of mnemonic names and operand types the assembler will match against using parse_operand(). It may also include a target specific mnemonic_extension.

char *cpu_copyright;

A string that will be emitted as part of the copyright message.

char *cpuname;

A string describing the target cpu.

int init_cpu();

Will be called during startup, after argument parsing. Must return zero if initializations failed, non-zero otherwise.

int cpu_args(char *);

This function will be called with the command line arguments (unless they were already recognized by other modules). If an argument was recognized, return non-zero.

char *parse_cpu_special(char *);

This function will be called with a source line as argument and allows the cpu module to handle cpu-specific directives etc. Functions like eol() and skip() should be used by the syntax module to keep the syntax consistent.

operand *new_operand();

Allocate and initialize a new operand structure.

int parse_operand(char *text,int len,operand *out,int requires);

Parses the source at text with length len to fill the target specific operand structure pointed to by out. Returns PO_MATCH when the operand matches the operand-type passed in requires and PO_NOMATCH otherwise. When the source is definitely identified as garbage, the function may return PO_CORRUPT to tell the assembler that it is useless to try matching against any other operand types. Another special case is PO_SKIP, which is also a match, but skips the next operand from the mnemonic table (because it was already handled together with the current operand).

taddr instruction_size(instruction *ip, section *sec, taddr pc);

Returns the size of the instruction ip in bytes, which must be identical to the number of bytes written by eval_instruction() (see below).

dblock *eval_instruction(instruction *ip, section *sec, taddr pc);

Converts the instruction ip into a DATA atom, including relocations, if necessary.

dblock *eval_data(operand *op, taddr bitsize, section *sec, taddr pc);

Converts a data operand into a DATA atom, including relocations.

void init_instruction_ext(instruction_ext *);

(If HAVE_INSTRUCTION_EXTENSION is set.) Initialize an instruction extension.

char *parse_instruction(char *,int *,char **,int *,int *);

(If MAX_QUALIFIERS is greater than 0.) Parses instruction and saves extension locations.

int set_default_qualifiers(char **,int *);

(If MAX_QUALIFIERS is greater than 0.) Saves pointers and lengths of default qualifiers for the selected CPU and returns the number of default qualifiers. Example: for a M680x0 CPU this would be a single qualifier, called "w". Used by execute_macro().

cpu_opts_init(section *);

(If HAVE_CPU_OPTS is set.) Gives the cpu module the chance to write out OPTS atoms with initial settings before the first atom is generated.

cpu_opts(void *);

(If HAVE_CPU_OPTS is set.) Apply option modifications from an OPTS atom. For example: change cpu type or optimization flags.

print_cpu_opts(FILE *,void *);

(If HAVE_CPU_OPTS is set.) Called from print_atom() to print an OPTS atom’s contents.

26.6 Output modules

Output modules can be chosen at runtime rather than compile time. Therefore, several output modules are linked into one vasm executable and their structure differs somewhat from syntax and cpu modules.

Usually, an output module for some object format fmt should be contained in a file ‘output_<fmt>.c’ (it may use/include other files if necessary). To automatically include this format in the build process, the ‘make.rules’ has to be extended. The module should be added to the OBJS variable at the start of ‘make.rules’. Also, a dependency line should be added (see the existing output modules).

An output module must only export a single function which will return pointers to necessary data/functions. This function should have the following prototype:

int init_output_<fmt>(
      char **copyright,
      void (**write_object)(FILE *,section *,symbol *),
      int (**output_args)(char *)

In case of an error, zero must be returned. Otherwise, It should perform all necessary initializations, return non-zero and return the following output parameters via the pointers passed as arguments:


A pointer to the copyright string.


A pointer to a function emitting the output. It will be called after the assembler has completed and will receive pointers to the output file, to the first section of the section list and to the first symbol in the symbol list. See the section on general data structures for further details.


A pointer to a function checking arguments. It will be called with all command line arguments (unless already handled by other modules). If the output module recognizes an appropriate option, it has to handle it and return non-zero. If it is not an option relevant to this output module, zero must be returned.

At last, a call to the output_init_<fmt> has to be added in the init_output() function in ‘vasm.c’ (should be self-explanatory).

Some remarks:

Volker Barthelmann vb@compilers.de

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