We write assembly with absolute addresses and offsets but in both jumps and branches it gets compiled to an offset. This is why you can't jump or branch to anywhere in memory, expect using the jump to register jr instruction.
This is because of a fundamental design of MIPS, fixed length one word instructions. They contain an id for instruction called op-code which is 6 bits along with other information needed to execute the instruction. This may be the id of registers or 'immediate' values, basically integers encoded in the instruction. To get to one of those bytes we need to specify the address.
If were lucky to store the address in a register we would just jr and use the address already stored in register. However, we are not. This becomes problematic, we only have 32 bits for our instructions and we would need all those bits to specify the address in that range. We also had to give up 6 bits to be used by the processor to identify the instruction.
Now we are left with 26 bits. What is worse is that when we branch, we need 10 additional bits to specify the two registers we are comparing for our condition.
The solution is to use offsets. Let's say that we are at address 0x and we are executing an unconditional jump to the next address in memory j 0xc. This is the assembly code and I'll show how this gets translated into machine code and executed. First we cheat a little. We know instructions are one word 4 bytes and in MIPS it's specified that they must be within the word boundary. This means that all instructions have addresses that are 4 bytes apart and this means they always end in 00 in binary representation.
Great, we can shave off those two meaningless bits. We also shave of the first 6, but don't worry, we'll get them back later. Which is 0xc in Hex and where we want to go, now we jump there. The same basic idea holds for branches, except now you also have the 2 registers being compared which require 10 bits so you only have 16 bits which you can use to offset, hence why you can't jump as far with branches.
Generally, this is fine because we mostly use branches within a procedure, to implement loops and carry out conditional assignments. This is all a consequence of the design of the MIPS architecture. It would have been entirely possible to have instructions where the only difference between branches and jumps would have been the conditional aspects and where an 'unconditional' branch would have behaved the same as an unconditional jump.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. What is the difference between unconditional branch and unconditional jump instructions in MIPS? Ask Question. Asked 9 years, 5 months ago. Active 3 years, 6 months ago. Viewed 70k times. Improve this question. Val Val 1. Add a comment. Active Oldest Votes. Improve this answer. Z80 has relative jumps, and conditional jumps, and conditional relative jumps.
A branch is a word nonexisting in Z80 assembly. So, if I am not interested in a condition, jump is always better? HelloWorld Typically? But always? One situation I can think of is in a device with a very small memory. In that case, you wouldn't have far to go and would want to save as many bits as possible.
So, an unconditional branch would be more appropriate. This fails to address the question of if there is any difference between unconditional branch and unconditional jump. Show 1 more comment. Igor Skochinsky Igor Skochinsky Call Instructions — The call instruction transfers the program sequence to the memory address given in the operand. Before transferring, the address of the next instruction after CALL is pushed onto the stack.
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