Transcript Class 3.23 MIPS Assembly Syntax. Example Programx

MIPS assembly syntax
Comments
The character "#" (sharp sign) starts a comment;
 Everything on the line from "#" to the right is ignored.
 Sometimes you can use two or more in a row for
emphasis, but only one is needed.
## Program to add two plus three -
is a
comment. It is ignored by the assembler and results in no
machine instructions.
Identifiers
Identifiers are a sequence of alphanumeric characters,
underbars (_), and dots (.) that do not begin with a number.
Directives
A directive is a statement that tells the assembler
something about what the programmer wants, but does not
itself result in any machine instructions.
 .text
Subsequent items put in user text segment
(machine code)
 .data
Subsequent items put in user data segment
(binary rep of data in source file)
 .globl sym
Declares sym global and can be
referenced from other files
 .asciiz str
Store the string str in memory and nullterminate it
 .byte b1…bn Store the n values in successive
memory bytes.
 .word w1…wn Store the n 32-bit quantities in
successive memory words
An example program
.text is a directive.
This directive tells the assembler that
• the following lines are instructions
• and should produce machine instructions
• and put into the binary code text segment.
For simple programs
.text and .globl could
be omitted.
.globl main is another directive.
## Program to add two plus
three
It says
that
.text
• the identifier main will be used outside of this
source file (that is, used "globally")
.globl main
main:
ori $8, $0, 0x2
ori $9, $0, 3
add $10, $8, $9
## End of file
• as the label of a particular location in main
memory.
# put the number two into register 8
# put the number three into register 9
# add register 8 and 9, put result in 10
The blank line is ignored.
Labels
Labels are declared by putting them at the beginning of a line followed by
a colon, for example:
main:
ori
$8,$0,0x2
main: symbolic address or a statement label.
A symbolic address is a symbol (an identifier followed by a colon ``:'') that is
the source code name for a location in memory.
In this program, main stands for the address of the first machine
instruction (which turns out to be 0x00400000).
## Program to add two plus three
.text
.globl main
main:
ori $8, $0, 0x2
ori $9, $0, 3
add $10, $8, $9
## End of file
# put the number two into register 8
# put the number three into register 9
# add register 8 and 9, put result in 10
Opcodes
Instruction opcodes are reserved words that cannot be used
as identifiers
ori, add – are operation codes (opcodes) of assembly
instructions.
.text
.globl main
main:
ori $8, $0, 0x2
ori $9, $0, 3
add $10, $8, $9
# put the number two into register 8
# put the number three into register 9
# add register 8 and 9, put result in 10
Machine Instructions
• Each of the three lines following main: corresponds to one machine
instruction.
• The line: ori $8,$0,0x2 is translated into a 32-bit machine
instruction.
• The machine instruction, upon execution, puts a 32-bit two's
complement positive 2 into register 8(details later).
• The instruction after that is translated into a machine instruction that
(upon execution) puts a 3 into register 9.
• The final instruction is translated into a machine instruction that (upon
execution) adds register 8 to register 9 and puts the 32-bit result into
register 10.
.text
.globl main
main:
ori $8, $0, 0x2
ori $9, $0, 3
add $10, $8, $9
# put the number two into register 8
# put the number three into register 9
# add register 8 and 9, put result in 10
Numbers
• Numbers are base 10 by default.
• If they are preceded by 0x, they are interpreted as
hexadecimal.
• 256 and 0x100 denote the same value.
ori $8,$0,0x100
ori $8,$0,256
.text
.globl main
main:
ori $8, $0, 0x2
ori $9, $0, 3
add $10, $8, $9
# put the number two into register 8
# put the number three into register 9
# add register 8 and 9, put result in 10
Strings, Data
Strings are enclosed in double quotes (").
.data
string: .asciiz
.asciiz str
"Hello SPIM!\n\n"
- Stores the string str in memory and null-terminates it.
Special characters in strings follow the C convention:
 newline \n
 tab \t
 quote \"
.data is a directive. This directive tells the assembler that
the subsequent items should be stored in the data segment of
binary code.
Examples on the AUA server