174

Chapter 30. MMIX Dependent Features

30.2. Instruction expansion

When

as

encounters an instruction with an operand that is either not known or does not fit the operand

size of the instruction,

as

(and

ld

) will expand the instruction into a sequence of instructions seman 

tically equivalent to the operand fitting the instruction. Expansion will take place for the following

instructions:

GETA

Expands to a sequence of four instructions:

SETL

,

INCML

,

INCMH

and

INCH

. The operand must

be a multiple of four.

Conditional branches

A branch instruction is turned into a branch with the complemented condition and prediction bit

over five instructions; four instructions setting

$255

to the operand value, which like with

GETA

must be a multiple of four, and a final

GO $255,$255,0

.

PUSHJ

Similar to expansion for conditional branches; four instructions set

$255

to the operand value,

followed by a

PUSHGO $255,$255,0

.

JMP

Similar to conditional branches and

PUSHJ

. The final instruction is

GO $255,$255,0

.

The linker

ld

is expected to shrink these expansions for code assembled with

 relax

(though not

currently implemented).

30.3. Syntax

The assembly syntax is supposed to be upward compatible with that described in Sections 1.3 and 1.4

of

The Art of Computer Programming, Volume 1

. Draft versions of those chapters as well

as other MMIX information is located at 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 (Section 30.4 Differences to

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

0x

, a hexadecimal number can also be specified by the prefix

character

#

.

After all operands to an MMIX instruction or directive have been specified, the rest of the line is

ignored, treated as a comment.

30.3.1. Special Characters

The characters

*

and

#

are line comment characters; each start a comment at the beginning of a line,

but only at the beginning of a line. A

#

prefixes a hexadecimal number if found elsewhere on a line.

Two other characters,

%

and

!

, each start a comment anywhere on the line. Thus you can't use the

modulus

and

not

operators in expressions normally associated with these two characters.

A

;

is a line separator, treated as a new line, so separate instructions can be specified on a single line.