Hello World Silly Example
.globl main
.text
main: # object1.print();
la $a0,object1 # get address of first object
lw $t0,0($a0) # get address of object's method
jalr $t0 # call the object's method
# object2.print();
la $a0,object2 # get address of second object
lw $t0,0($a0) # get address of object's method
jalr $t0 # call the object's method
li $v0,10 # return to OS
syscall
# code for print method
. . .
.data
object1: .word print # Jump Table
.asciiz "Hello World\n" # object data
object2: .word print # Jump Table
.asciiz "Silly Example\n" # object's data
In Java,
objects are constructed from dynamic memory as the program runs.
However, to simplify the discussion (and the code)
this example uses
static memory for objects.
This means that the objects already exist
in the .data section
in memory when the program starts running.
The program uses two objects in static memory declared in the source file.
Each object is implemented like this:
byte 0-3: address of a method # single-entry jump table byte 4- : null terminated string # as many bytes as needed
The jump table contains just one address,
the entry point of print(),
at the start of the object.
The following code copies this address from object1
to $t0:
la $a0,object1 # get address of first object lw $t0,0($a0) # get address of object's method
Now the entry point is in $t0.
Then object1's method is called.
Both objects contain the same address for the method. The program has only one copy of the code for the method.
What instruction is used to call object1's method?