gas/doc/c-mmix.texi

@c Copyright (C) 2001-2021 Free Software Foundation, Inc.
@c This is part of the GAS manual.
@c For copying conditions, see the file as.texinfo.
@c MMIX description by Hans-Peter Nilsson, hp@bitrange.com
@ifset GENERIC
@page
@node MMIX-Dependent
@chapter MMIX Dependent Features
@end ifset
@ifclear GENERIC
@node Machine Dependencies
@chapter MMIX Dependent Features
@end ifclear

@cindex MMIX support
@menu
* MMIX-Opts::              Command-line Options
* MMIX-Expand::            Instruction expansion
* MMIX-Syntax::            Syntax
* MMIX-mmixal::		   Differences to @code{mmixal} syntax and semantics
@end menu

@node MMIX-Opts
@section Command-line Options

@cindex options, MMIX
@cindex MMIX options
The MMIX version of @code{@value{AS}} has some machine-dependent options.

@cindex @samp{--fixed-special-register-names} command-line option, MMIX
When @samp{--fixed-special-register-names} is specified, only the register
names specified in @ref{MMIX-Regs} are recognized in the instructions
@code{PUT} and @code{GET}.

@cindex @samp{--globalize-symbols} command-line option, MMIX
You can use the @samp{--globalize-symbols} to make all symbols global.
This option is useful when splitting up a @code{mmixal} program into
several files.

@cindex @samp{--gnu-syntax} command-line option, MMIX
The @samp{--gnu-syntax} turns off most syntax compatibility with
@code{mmixal}.  Its usability is currently doubtful.

@cindex @samp{--relax} command-line option, MMIX
The @samp{--relax} option is not fully supported, but will eventually make
the object file prepared for linker relaxation.

@cindex @samp{--no-predefined-syms} command-line option, MMIX
If you want to avoid inadvertently calling a predefined symbol and would
rather get an error, for example when using @code{@value{AS}} with a
compiler or other machine-generated code, specify
@samp{--no-predefined-syms}.  This turns off built-in predefined
definitions of all such symbols, including rounding-mode symbols, segment
symbols, @samp{BIT} symbols, and @code{TRAP} symbols used in @code{mmix}
``system calls''.  It also turns off predefined special-register names,
except when used in @code{PUT} and @code{GET} instructions.

@cindex @samp{--no-expand} command-line option, MMIX
By default, some instructions are expanded to fit the size of the operand
or an external symbol (@pxref{MMIX-Expand}).  By passing
@samp{--no-expand}, no such expansion will be done, instead causing errors
at link time if the operand does not fit.

@cindex @samp{--no-merge-gregs} command-line option, MMIX
The @code{mmixal} documentation (@pxref{mmixsite}) specifies that global
registers allocated with the @samp{GREG} directive (@pxref{MMIX-greg}) and
initialized to the same non-zero value, will refer to the same global
register.  This isn't strictly enforceable in @code{@value{AS}} since the
final addresses aren't known until link-time, but it will do an effort
unless the @samp{--no-merge-gregs} option is specified.  (Register merging
isn't yet implemented in @code{@value{LD}}.)

@cindex @samp{-x} command-line option, MMIX
@code{@value{AS}} will warn every time it expands an instruction to fit an
operand unless the option @samp{-x} is specified.  It is believed that
this behaviour is more useful than just mimicking @code{mmixal}'s
behaviour, in which instructions are only expanded if the @samp{-x} option
is specified, and assembly fails otherwise, when an instruction needs to
be expanded.  It needs to be kept in mind that @code{mmixal} is both an
assembler and linker, while @code{@value{AS}} will expand instructions
that at link stage can be contracted.  (Though linker relaxation isn't yet
implemented in @code{@value{LD}}.)  The option @samp{-x} also implies
@samp{--linker-allocated-gregs}.

@cindex @samp{--no-pushj-stubs} command-line option, MMIX
@cindex @samp{--no-stubs} command-line option, MMIX
If instruction expansion is enabled, @code{@value{AS}} can expand a
@samp{PUSHJ} instruction into a series of instructions.  The shortest
expansion is to not expand it, but just mark the call as redirectable to a
stub, which @code{@value{LD}} creates at link-time, but only if the
original @samp{PUSHJ} instruction is found not to reach the target.  The
stub consists of the necessary instructions to form a jump to the target.
This happens if @code{@value{AS}} can assert that the @samp{PUSHJ}
instruction can reach such a stub.  The option @samp{--no-pushj-stubs}
disables this shorter expansion, and the longer series of instructions is
then created at assembly-time.  The option @samp{--no-stubs} is a synonym,
intended for compatibility with future releases, where generation of stubs
for other instructions may be implemented.

@cindex @samp{--linker-allocated-gregs} command-line option, MMIX
Usually a two-operand-expression (@pxref{GREG-base}) without a matching
@samp{GREG} directive is treated as an error by @code{@value{AS}}.  When
the option @samp{--linker-allocated-gregs} is in effect, they are instead
passed through to the linker, which will allocate as many global registers
as is needed.

@node MMIX-Expand
@section Instruction expansion

@cindex instruction expansion, MMIX
When @code{@value{AS}} encounters an instruction with an operand that is
either not known or does not fit the operand size of the instruction,
@code{@value{AS}} (and @code{@value{LD}}) will expand the instruction into
a sequence of instructions semantically equivalent to the operand fitting
the instruction.  Expansion will take place for the following
instructions:

@table @asis
@item @samp{GETA}
Expands to a sequence of four instructions: @code{SETL}, @code{INCML},
@code{INCMH} and @code{INCH}.  The operand must be a multiple of four.
@item Conditional branches
A branch instruction is turned into a branch with the complemented
condition and prediction bit over five instructions; four instructions
setting @code{$255} to the operand value, which like with @code{GETA} must
be a multiple of four, and a final @code{GO $255,$255,0}.
@item @samp{PUSHJ}
Similar to expansion for conditional branches; four instructions set
@code{$255} to the operand value, followed by a @code{PUSHGO $255,$255,0}.
@item @samp{JMP}
Similar to conditional branches and @code{PUSHJ}.  The final instruction
is @code{GO $255,$255,0}.
@end table

The linker @code{@value{LD}} is expected to shrink these expansions for
code assembled with @samp{--relax} (though not currently implemented).

@node MMIX-Syntax
@section Syntax

The assembly syntax is supposed to be upward compatible with that
described in Sections 1.3 and 1.4 of @samp{The Art of Computer
Programming, Volume 1}.  Draft versions of those chapters as well as other
MMIX information is located at
@anchor{mmixsite}@url{http://www-cs-faculty.stanford.edu/~knuth/mmix-news.html}.
Most code examples from the mmixal package located there should work
unmodified when assembled and linked as single files, with a few
noteworthy exceptions (@pxref{MMIX-mmixal}).

Before an instruction is emitted, the current location is aligned to the
next four-byte boundary.  If a label is defined at the beginning of the
line, its value will be the aligned value.

In addition to the traditional hex-prefix @samp{0x}, a hexadecimal number
can also be specified by the prefix character @samp{#}.

After all operands to an MMIX instruction or directive have been
specified, the rest of the line is ignored, treated as a comment.

@menu
* MMIX-Chars::		        Special Characters
* MMIX-Symbols::		Symbols
* MMIX-Regs::			Register Names
* MMIX-Pseudos::		Assembler Directives
@end menu

@node MMIX-Chars
@subsection Special Characters
@cindex line comment characters, MMIX
@cindex MMIX line comment characters

The characters @samp{*} and @samp{#} are line comment characters; each
start a comment at the beginning of a line, but only at the beginning of a
line.  A @samp{#} prefixes a hexadecimal number if found elsewhere on a
line.  If a @samp{#} appears at the start of a line the whole line is
treated as a comment, but the line can also act as a logical line
number directive (@pxref{Comments}) or a preprocessor control command
(@pxref{Preprocessing}).

Two other characters, @samp{%} and @samp{!}, each start a comment anywhere
on the line.  Thus you can't use the @samp{modulus} and @samp{not}
operators in expressions normally associated with these two characters.

A @samp{;} is a line separator, treated as a new-line, so separate
instructions can be specified on a single line.

@node MMIX-Symbols
@subsection Symbols
The character @samp{:} is permitted in identifiers.  There are two
exceptions to it being treated as any other symbol character: if a symbol
begins with @samp{:}, it means that the symbol is in the global namespace
and that the current prefix should not be prepended to that symbol
(@pxref{MMIX-prefix}).  The @samp{:} is then not considered part of the
symbol.  For a symbol in the label position (first on a line), a @samp{:}
at the end of a symbol is silently stripped off.  A label is permitted,
but not required, to be followed by a @samp{:}, as with many other
assembly formats.

The character @samp{@@} in an expression, is a synonym for @samp{.}, the
current location.

In addition to the common forward and backward local symbol formats
(@pxref{Symbol Names}), they can be specified with upper-case @samp{B} and
@samp{F}, as in @samp{8B} and @samp{9F}.  A local label defined for the
current position is written with a @samp{H} appended to the number:
@smallexample
3H LDB $0,$1,2
@end smallexample
This and traditional local-label formats cannot be mixed: a label must be
defined and referred to using the same format.

There's a minor caveat: just as for the ordinary local symbols, the local
symbols are translated into ordinary symbols using control characters are
to hide the ordinal number of the symbol.  Unfortunately, these symbols
are not translated back in error messages.  Thus you may see confusing
error messages when local symbols are used.  Control characters
@samp{\003} (control-C) and @samp{\004} (control-D) are used for the
MMIX-specific local-symbol syntax.

The symbol @samp{Main} is handled specially; it is always global.

By defining the symbols @samp{__.MMIX.start..text} and
@samp{__.MMIX.start..data}, the address of respectively the @samp{.text}
and @samp{.data} segments of the final program can be defined, though when
linking more than one object file, the code or data in the object file
containing the symbol is not guaranteed to be start at that position; just
the final executable.  @xref{MMIX-loc}.

@node MMIX-Regs
@subsection Register names
@cindex register names, MMIX
@cindex MMIX register names

Local and global registers are specified as @samp{$0} to @samp{$255}.
The recognized special register names are @samp{rJ}, @samp{rA}, @samp{rB},
@samp{rC}, @samp{rD}, @samp{rE}, @samp{rF}, @samp{rG}, @samp{rH},
@samp{rI}, @samp{rK}, @samp{rL}, @samp{rM}, @samp{rN}, @samp{rO},
@samp{rP}, @samp{rQ}, @samp{rR}, @samp{rS}, @samp{rT}, @samp{rU},
@samp{rV}, @samp{rW}, @samp{rX}, @samp{rY}, @samp{rZ}, @samp{rBB},
@samp{rTT}, @samp{rWW}, @samp{rXX}, @samp{rYY} and @samp{rZZ}.  A leading
@samp{:} is optional for special register names.

Local and global symbols can be equated to register names and used in
place of ordinary registers.

Similarly for special registers, local and global symbols can be used.
Also, symbols equated from numbers and constant expressions are allowed in
place of a special register, except when either of the options
@code{--no-predefined-syms} and @code{--fixed-special-register-names} are
specified.  Then only the special register names above are allowed for the
instructions having a special register operand; @code{GET} and @code{PUT}.

@node MMIX-Pseudos
@subsection Assembler Directives
@cindex assembler directives, MMIX
@cindex pseudo-ops, MMIX
@cindex MMIX assembler directives
@cindex MMIX pseudo-ops

@table @code
@item LOC
@cindex assembler directive LOC, MMIX
@cindex pseudo-op LOC, MMIX
@cindex MMIX assembler directive LOC
@cindex MMIX pseudo-op LOC

@anchor{MMIX-loc}
The @code{LOC} directive sets the current location to the value of the
operand field, which may include changing sections.  If the operand is a
constant, the section is set to either @code{.data} if the value is
@code{0x2000000000000000} or larger, else it is set to @code{.text}.
Within a section, the current location may only be changed to
monotonically higher addresses.  A LOC expression must be a previously
defined symbol or a ``pure'' constant.

An example, which sets the label @var{prev} to the current location, and
updates the current location to eight bytes forward:
@smallexample
prev LOC @@+8
@end smallexample

When a LOC has a constant as its operand, a symbol
@code{__.MMIX.start..text} or @code{__.MMIX.start..data} is defined
depending on the address as mentioned above.  Each such symbol is
interpreted as special by the linker, locating the section at that
address.  Note that if multiple files are linked, the first object file
with that section will be mapped to that address (not necessarily the file
with the LOC definition).

@item LOCAL
@cindex assembler directive LOCAL, MMIX
@cindex pseudo-op LOCAL, MMIX
@cindex MMIX assembler directive LOCAL
@cindex MMIX pseudo-op LOCAL

@anchor{MMIX-local}
Example:
@smallexample
 LOCAL external_symbol
 LOCAL 42
 .local asymbol
@end smallexample

This directive-operation generates a link-time assertion that the operand
does not correspond to a global register.  The operand is an expression
that at link-time resolves to a register symbol or a number.  A number is
treated as the register having that number.  There is one restriction on
the use of this directive: the pseudo-directive must be placed in a
section with contents, code or data.

@item IS
@cindex assembler directive IS, MMIX
@cindex pseudo-op IS, MMIX
@cindex MMIX assembler directive IS
@cindex MMIX pseudo-op IS

@anchor{MMIX-is}
The @code{IS} directive:
@smallexample
asymbol IS an_expression
@end smallexample
sets the symbol @samp{asymbol} to @samp{an_expression}.  A symbol may not
be set more than once using this directive.  Local labels may be set using
this directive, for example:
@smallexample
5H IS @@+4
@end smallexample

@item GREG
@cindex assembler directive GREG, MMIX
@cindex pseudo-op GREG, MMIX
@cindex MMIX assembler directive GREG
@cindex MMIX pseudo-op GREG

@anchor{MMIX-greg}
This directive reserves a global register, gives it an initial value and
optionally gives it a symbolic name.  Some examples:

@smallexample
areg GREG
breg GREG data_value
     GREG data_buffer
     .greg creg, another_data_value
@end smallexample

The symbolic register name can be used in place of a (non-special)
register.  If a value isn't provided, it defaults to zero.  Unless the
option @samp{--no-merge-gregs} is specified, non-zero registers allocated
with this directive may be eliminated by @code{@value{AS}}; another
register with the same value used in its place.
Any of the instructions
@samp{CSWAP},
@samp{GO},
@samp{LDA},
@samp{LDBU},
@samp{LDB},
@samp{LDHT},
@samp{LDOU},
@samp{LDO},
@samp{LDSF},
@samp{LDTU},
@samp{LDT},
@samp{LDUNC},
@samp{LDVTS},
@samp{LDWU},
@samp{LDW},
@samp{PREGO},
@samp{PRELD},
@samp{PREST},
@samp{PUSHGO},
@samp{STBU},
@samp{STB},
@samp{STCO},
@samp{STHT},
@samp{STOU},
@samp{STSF},
@samp{STTU},
@samp{STT},
@samp{STUNC},
@samp{SYNCD},
@samp{SYNCID},
can have a value nearby @anchor{GREG-base}an initial value in place of its
second and third operands.  Here, ``nearby'' is defined as within the
range 0@dots{}255 from the initial value of such an allocated register.

@smallexample
buffer1 BYTE 0,0,0,0,0
buffer2 BYTE 0,0,0,0,0
 @dots{}
 GREG buffer1
 LDOU $42,buffer2
@end smallexample
In the example above, the @samp{Y} field of the @code{LDOUI} instruction
(LDOU with a constant Z) will be replaced with the global register
allocated for @samp{buffer1}, and the @samp{Z} field will have the value
5, the offset from @samp{buffer1} to @samp{buffer2}.  The result is
equivalent to this code:
@smallexample
buffer1 BYTE 0,0,0,0,0
buffer2 BYTE 0,0,0,0,0
 @dots{}
tmpreg GREG buffer1
 LDOU $42,tmpreg,(buffer2-buffer1)
@end smallexample

Global registers allocated with this directive are allocated in order
higher-to-lower within a file.  Other than that, the exact order of
register allocation and elimination is undefined.  For example, the order
is undefined when more than one file with such directives are linked
together.  With the options @samp{-x} and @samp{--linker-allocated-gregs},
@samp{GREG} directives for two-operand cases like the one mentioned above
can be omitted.  Sufficient global registers will then be allocated by the
linker.

@item BYTE
@cindex assembler directive BYTE, MMIX
@cindex pseudo-op BYTE, MMIX
@cindex MMIX assembler directive BYTE
@cindex MMIX pseudo-op BYTE

@anchor{MMIX-byte}
The @samp{BYTE} directive takes a series of operands separated by a comma.
If an operand is a string (@pxref{Strings}), each character of that string
is emitted as a byte.  Other operands must be constant expressions without
forward references, in the range 0@dots{}255.  If you need operands having
expressions with forward references, use @samp{.byte} (@pxref{Byte}).  An
operand can be omitted, defaulting to a zero value.

@item WYDE
@itemx TETRA
@itemx OCTA
@cindex assembler directive WYDE, MMIX
@cindex pseudo-op WYDE, MMIX
@cindex MMIX assembler directive WYDE
@cindex MMIX pseudo-op WYDE
@cindex assembler directive TETRA, MMIX
@cindex pseudo-op TETRA, MMIX
@cindex MMIX assembler directive TETRA
@cindex MMIX pseudo-op TETRA
@cindex assembler directive OCTA, MMIX
@cindex pseudo-op OCTA, MMIX
@cindex MMIX assembler directive OCTA
@cindex MMIX pseudo-op OCTA

@anchor{MMIX-constants}
The directives @samp{WYDE}, @samp{TETRA} and @samp{OCTA} emit constants of
two, four and eight bytes size respectively.  Before anything else happens
for the directive, the current location is aligned to the respective
constant-size boundary.  If a label is defined at the beginning of the
line, its value will be that after the alignment.  A single operand can be
omitted, defaulting to a zero value emitted for the directive.  Operands
can be expressed as strings (@pxref{Strings}), in which case each
character in the string is emitted as a separate constant of the size
indicated by the directive.

@item PREFIX
@cindex assembler directive PREFIX, MMIX
@cindex pseudo-op PREFIX, MMIX
@cindex MMIX assembler directive PREFIX
@cindex MMIX pseudo-op PREFIX

@anchor{MMIX-prefix}
The @samp{PREFIX} directive sets a symbol name prefix to be prepended to
all symbols (except local symbols, @pxref{MMIX-Symbols}), that are not
prefixed with @samp{:}, until the next @samp{PREFIX} directive.  Such
prefixes accumulate.  For example,
@smallexample
 PREFIX a
 PREFIX b
c IS 0
@end smallexample
defines a symbol @samp{abc} with the value 0.

@item BSPEC
@itemx ESPEC
@cindex assembler directive BSPEC, MMIX
@cindex pseudo-op BSPEC, MMIX
@cindex MMIX assembler directive BSPEC
@cindex MMIX pseudo-op BSPEC
@cindex assembler directive ESPEC, MMIX
@cindex pseudo-op ESPEC, MMIX
@cindex MMIX assembler directive ESPEC
@cindex MMIX pseudo-op ESPEC

@anchor{MMIX-spec}
A pair of @samp{BSPEC} and @samp{ESPEC} directives delimit a section of
special contents (without specified semantics).  Example:
@smallexample
 BSPEC 42
 TETRA 1,2,3
 ESPEC
@end smallexample
The single operand to @samp{BSPEC} must be number in the range
0@dots{}255.  The @samp{BSPEC} number 80 is used by the GNU binutils
implementation.
@end table

@node MMIX-mmixal
@section Differences to @code{mmixal}
@cindex mmixal differences
@cindex differences, mmixal

The binutils @code{@value{AS}} and @code{@value{LD}} combination has a few
differences in function compared to @code{mmixal} (@pxref{mmixsite}).

The replacement of a symbol with a GREG-allocated register
(@pxref{GREG-base}) is not handled the exactly same way in
@code{@value{AS}} as in @code{mmixal}.  This is apparent in the
@code{mmixal} example file @code{inout.mms}, where different registers
with different offsets, eventually yielding the same address, are used in
the first instruction.  This type of difference should however not affect
the function of any program unless it has specific assumptions about the
allocated register number.

Line numbers (in the @samp{mmo} object format) are currently not
supported.

Expression operator precedence is not that of mmixal: operator precedence
is that of the C programming language.  It's recommended to use
parentheses to explicitly specify wanted operator precedence whenever more
than one type of operators are used.

The serialize unary operator @code{&}, the fractional division operator
@samp{//}, the logical not operator @code{!} and the modulus operator
@samp{%} are not available.

Symbols are not global by default, unless the option
@samp{--globalize-symbols} is passed.  Use the @samp{.global} directive to
globalize symbols (@pxref{Global}).

Operand syntax is a bit stricter with @code{@value{AS}} than
@code{mmixal}.  For example, you can't say @code{addu 1,2,3}, instead you
must write @code{addu $1,$2,3}.

You can't LOC to a lower address than those already visited
(i.e., ``backwards'').

A LOC directive must come before any emitted code.

Predefined symbols are visible as file-local symbols after use.  (In the
ELF file, that is---the linked mmo file has no notion of a file-local
symbol.)

Some mapping of constant expressions to sections in LOC expressions is
attempted, but that functionality is easily confused and should be avoided
unless compatibility with @code{mmixal} is required.  A LOC expression to
@samp{0x2000000000000000} or higher, maps to the @samp{.data} section and
lower addresses map to the @samp{.text} section (@pxref{MMIX-loc}).

The code and data areas are each contiguous.  Sparse programs with
far-away LOC directives will take up the same amount of space as a
contiguous program with zeros filled in the gaps between the LOC
directives.  If you need sparse programs, you might try and get the wanted
effect with a linker script and splitting up the code parts into sections
(@pxref{Section}).  Assembly code for this, to be compatible with
@code{mmixal}, would look something like:
@smallexample
 .if 0
 LOC away_expression
 .else
 .section away,"ax"
 .fi
@end smallexample
@code{@value{AS}} will not execute the LOC directive and @code{mmixal}
ignores the lines with @code{.}.  This construct can be used generally to
help compatibility.

Symbols can't be defined twice--not even to the same value.

Instruction mnemonics are recognized case-insensitive, though the
@samp{IS} and @samp{GREG} pseudo-operations must be specified in
upper-case characters.

There's no unicode support.

The following is a list of programs in @samp{mmix.tar.gz}, available at
@url{http://www-cs-faculty.stanford.edu/~knuth/mmix-news.html}, last
checked with the version dated 2001-08-25 (md5sum
c393470cfc86fac040487d22d2bf0172) that assemble with @code{mmixal} but do
not assemble with @code{@value{AS}}:

@table @code
@item silly.mms
LOC to a previous address.
@item sim.mms
Redefines symbol @samp{Done}.
@item test.mms
Uses the serial operator @samp{&}.
@end table