Chapter 30. MMIX Dependent Features

175

30.3.2. Symbols

The character

:

is permitted in identifiers. There are two exceptions to it being treated as any other

symbol character: if a symbol begins with

:

, it means that the symbol is in the global namespace and

that the current prefix should not be prepended to that symbol. The

:

is then not considered part of

the symbol. For a symbol in the label position (first on a line), a

:

at the end of a symbol is silently

stripped off. A label is permitted, but not required, to be followed by a

:

, as with many other assembly

formats.

The character

@

in an expression, is a synonym for

.

, the current location.

In addition to the common forward and backward local symbol formats (Section 6.3 Symbol Names),

they can be specified with upper case

B

and

F

, as in

8B

and

9F

. A local label defined for the current

position is written with a

H

appended to the number:

3H LDB $0,$1,2

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

\003

(control C) and

\004

(control D) are used for

the MMIX specific local symbol syntax.

The symbol

Main

is handled specially; it is always global.

By defining the symbols

__.MMIX.start..text

and

__.MMIX.start..data

, the address of re 

spectively the

.text

and

.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.

30.3.3. Register names

Local and global registers are specified as

$0

to

$255

. The recognized special register names are

rJ

,

rA

,

rB

,

rC

,

rD

,

rE

,

rF

,

rG

,

rH

,

rI

,

rK

,

rL

,

rM

,

rN

,

rO

,

rP

,

rQ

,

rR

,

rS

,

rT

,

rU

,

rV

,

rW

,

rX

,

rY

,

rZ

,

rBB

,

rTT

,

rWW

,

rXX

,

rYY

and

rZZ

. A leading

:

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

 no predefined syms

and

 fixed special register names

are specified. Then

only the special register names above are allowed for the instructions having a special register operand;

GET

and

PUT

.

30.3.4. Assembler Directives

LOC

The

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

.data

if the value is

0x2000000000000000

or larger, else it is set to

.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.