MIPS 32bit random integer

Question

I am trying to get MIPS to generate 32bit random integers. I'm trying to use the following code but it gives an error when I run the program.

addi $v0, $zero, 42
li $a1, 4294967295 #32 bit number
syscall

I'm using MARS simulator. The program assembles without any errors. Problem occurs when the syscall above is executed. I'd be really grateful for any help.

Since it uses Java's random number support, it probably only supports signed numbers. I would recommend trying a number less than 2^31. — Gabe
@MarcB it says that the upper bound of range can not be negative. — harbinger
@Gabe yes, the program accepts 2^31, but I want the program to give me a random value between 0 and 2^32. So i somehow need to tell the program that the range I'm providing is unsigned. How can I do that? — harbinger

gusbro gusbro · Accepted Answer · 2013-03-27T18:59:09

I think the problem lies in the maximum number you provided (0xffffffff). Try using the (signed) maximum positive value (0x7fffffff)

addi $v0, $zero, 42
li $a1, 0x7fffffff  #32 bit number (maximum 32bit signed positive number)
syscall

As per OPs comment, to get a 32 bit number, you might do it in two steps: first "toss a coin" to get the 32th bit, and the randomly get the other 31 bits:

    addi $v0, $zero, 42
    move $a2, $zero
    li $a1, 2
    syscall
    bnez $a0, skip
    li $a2, 0x80000000
skip:
    li $a1, 0x7fffffff #31 bit number (maximum 32bit signed positive number)
    syscall
    or $a0, $a0, $a2   # Here $a0 has a 32 bit number

MIPS 32bit random integer

1 Answers