CODEWITHSUNDEEP
assemblymips
user5600232
user5600232

Reputation:

MIPS Assembly Return to Call in a branch statement

I have:

CODE

beq $s3, 1, option1
beq $s3, 0, option2


MORE CODE

option1:
    code
    jr $ra????

option2:
    code
    jr $ra

I am trying to test s3 if it is a 0 or a 1 and run a different block of code (outputting a string) then continue with the rest of the code starting from immediately after the second beq

If I do it with jr $ra in each statement, I get an infinite loop. If I do it without those, it runs both statements then skips the rest of the code block im in and continues to the next function..

How can I make a branch statement that will bring me back to it when it is done?

Upvotes: 3

Views: 21391

Answers (2)

Zbynek Vyskovsky - kvr000
Zbynek Vyskovsky - kvr000

Reputation: 18825

I think what you actually need here, is regular jump instruction which jumps to instruction after the original switch-case:

    CODE

    beq $s3, 1, option1
    beq $s3, 0, option2
return_here:
    MORE CODE

option1:
    code
    j return_here

option2:
    code
    j return_here

This will jump to option1 or option2 and when finished it will jump to return_here label. The jr $ra instruction returns from a subroutine which means that the code would have to be invoked with jal instruction (storing the current instruction pointer to $ra and jumping to address).

Upvotes: 4

BiN4RY
BiN4RY

Reputation: 366

To use use the jump-return (jr) instruction, you must first make a jump using the jump-and-link instruction (jal). This instruction saves the program counter located in the $ra register before jumping to another part of the code. This way, the CPU knows which part of the code to return to when jr is called.

Standard branching instructions (like beq) does not save the program counter, so the CPU doesn't know what part of the code to return to when you call jr unexpectedly. This is why your code goes into a loop when you call jr.

Alternatively, you can also avoid linked jumping all together:

CODE

beq $s3, 1, option1
option1_ret:    

beq $s3, 0, option2
option2_ret: 

MORE CODE

option1:
    code
    j option1_ret

option2:
    code
    j option2_ret

Upvotes: 8

Related Questions