as: MMIX-Pseudos
9.27.3.4 Assembler Directives
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'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.
An example, which sets the label PREV to the current location, and
updates the current location to eight bytes forward:
prev LOC @+8
When a LOC has a constant as its operand, a symbol
'__.MMIX.start..text' or '__.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).
'LOCAL'
Example:
LOCAL external_symbol
LOCAL 42
.local asymbol
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.
'IS'
The 'IS' directive:
asymbol IS an_expression
sets the symbol 'asymbol' to 'an_expression'. A symbol may not be
set more than once using this directive. Local labels may be set
using this directive, for example:
5H IS @+4
'GREG'
This directive reserves a global register, gives it an initial
value and optionally gives it a symbolic name. Some examples:
areg GREG
breg GREG data_value
GREG data_buffer
.greg creg, another_data_value
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 '--no-merge-gregs' is specified, non-zero registers
allocated with this directive may be eliminated by 'as'; another
register with the same value used in its place. Any of the
instructions 'CSWAP', 'GO', 'LDA', 'LDBU', 'LDB', 'LDHT', 'LDOU',
'LDO', 'LDSF', 'LDTU', 'LDT', 'LDUNC', 'LDVTS', 'LDWU', 'LDW',
'PREGO', 'PRELD', 'PREST', 'PUSHGO', 'STBU', 'STB', 'STCO', 'STHT',
'STOU', 'STSF', 'STTU', 'STT', 'STUNC', 'SYNCD', 'SYNCID', can have
a value nearby an initial value in place of its second and third
operands. Here, "nearby" is defined as within the range 0...255
from the initial value of such an allocated register.
buffer1 BYTE 0,0,0,0,0
buffer2 BYTE 0,0,0,0,0
...
GREG buffer1
LDOU $42,buffer2
In the example above, the 'Y' field of the 'LDOUI' instruction
(LDOU with a constant Z) will be replaced with the global register
allocated for 'buffer1', and the 'Z' field will have the value 5,
the offset from 'buffer1' to 'buffer2'. The result is equivalent
to this code:
buffer1 BYTE 0,0,0,0,0
buffer2 BYTE 0,0,0,0,0
...
tmpreg GREG buffer1
LDOU $42,tmpreg,(buffer2-buffer1)
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 '-x' and
'--linker-allocated-gregs', 'GREG' directives for two-operand cases
like the one mentioned above can be omitted. Sufficient global
registers will then be allocated by the linker.
'BYTE'
The 'BYTE' directive takes a series of operands separated by a
comma. If an operand is a string (
Strings), each character
of that string is emitted as a byte. Other operands must be
constant expressions without forward references, in the range
0...255. If you need operands having expressions with forward
references, use '.byte' (Byte). An operand can be omitted,
defaulting to a zero value.
'WYDE'
'TETRA'
'OCTA'
The directives 'WYDE', 'TETRA' and '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 (
Strings), in which case each character in the string is emitted
as a separate constant of the size indicated by the directive.
'PREFIX'
The 'PREFIX' directive sets a symbol name prefix to be prepended to
all symbols (except local symbols, MMIX-Symbols), that are
not prefixed with ':', until the next 'PREFIX' directive. Such
prefixes accumulate. For example,
PREFIX a
PREFIX b
c IS 0
defines a symbol 'abc' with the value 0.
'BSPEC'
'ESPEC'
A pair of 'BSPEC' and 'ESPEC' directives delimit a section of
special contents (without specified semantics). Example:
BSPEC 42
TETRA 1,2,3
ESPEC
The single operand to 'BSPEC' must be number in the range 0...255.
The 'BSPEC' number 80 is used by the GNU binutils implementation.
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