Assembler Syntax

Comments in assembler files begin with a sharp-sign (#). Everything from the sharp-sign to the end of the line is ignored.

Identifiers are a sequence of alphanumeric characters, underbars (_), and dots (.) that do not begin with a number. Opcodes for instructions are reserved words that are not valid identifiers. Labels are declared by putting them at the beginning of a line followed by a colon, for example:

        .data
  item: .word 1
        .text
        .globl main             # Must be global
  main: lw $t0, item

Strings are enclosed in double-quotes ("). Special characters in strings follow the C convention:

    newline        \n
    tab            \t
    quote          \"

SPIM supports a subset of the assembler directives provided by the MIPS assembler:

.align n
Align the next datum on a $2^n$ byte boundary. For example, .align 2 aligns the next value on a word boundary. .align 0 turns off automatic alignment of .half, .word, .float, and .double directives until the next .data or .kdata directive.

.ascii str
Store the string in memory, but do not null-terminate it.

.asciiz str
Store the string in memory and null-terminate it.

.byte b1, ..., bn
Store the $n$ values in successive bytes of memory.

.comm sym size
Allocate size bytes of data segment for symbol sym.

.data <addr>
The following data items should be stored in the data segment. If the optional argument addr is present, the items are stored beginning at address addr.

.double d1, ..., dn
Store the $n$ floating point double precision numbers in successive memory locations.

.extern sym size
Declare that the datum stored at sym is size bytes large and is a global symbol. This directive enables the assembler to store the datum in a portion of the data segment that is efficiently accessed via register $gp.

.float f1, ..., fn
Store the $n$ floating point single precision numbers in successive memory locations.

.globl sym
Declare that symbol sym is global and can be referenced from other files.

.half h1, ..., hn
Store the $n$ 16-bit quantities in successive memory halfwords.

.kdata <addr>
The following data items should be stored in the kernel data segment. If the optional argument addr is present, the items are stored beginning at address addr.

.ktext <addr>
The next items are put in the kernel text segment. In SPIM, these items may only be instructions or words (see the .word directive below). If the optional argument addr is present, the items are stored beginning at address addr.

.label sym
Declare that symbol sym is a label.

.lcomm sym size
Allocate size bytes for symbol sym in the portion of the data segment that can be accessed via register $gp.

.space n
Allocate $n$ bytes of space in the current segment (which must be the data segment in SPIM).

.set noat
Permit the program to refer to the $at register explicitly, and forbid SPIM from generating pseudoinstructions that modify $at.

.set at
Forbid the program from referring to the $at register explicitly, and permit SPIM to generate pseudoinstructions that modify $at (the default).

.text <addr>
The next items are put in the user text segment. In SPIM, these items may only be instructions or words (see the .word directive below). If the optional argument addr is present, the items are stored beginning at address addr.

.word w1, ..., wn
Store the $n$ 32-bit quantities in successive memory words.

SPIM does not distinguish various parts of the data segment (.data, .rdata, and .sdata).