Let's break this down clearly, since you're working with a non-standard MIPS instruction and trying to unpack the branch addressing logic.

1. What the Custom rpt $t2, loop Instruction Does #

First, let's parse the behavior you described:

if(R[rs]>0) R[rs]=R[rs]−1, PC=PC+4+BranchAddr

Here's a plain-English breakdown:

• The instruction targets register $t2 (this is the rs register referenced in the description) and uses loop as its branch target label.
• When executed, it first checks if the value stored in $t2 is greater than 0.
  • If true:
    1. Decrement the value in $t2 by 1 (so R[$t2] = R[$t2] - 1).
    2. Update the program counter (PC) to jump to the loop label. The calculation PC=PC+4+BranchAddr is how the hardware computes the loop address (we'll dive into this next).
  • If false: The instruction does nothing, and the program continues executing the next sequential instruction (since the PC would just increment by 4 as usual).

This is essentially a custom loop counter instruction—it combines the logic of decrementing a counter and branching back to the loop start into a single step, similar to pairing a standard MIPS bgt (branch if greater than) and addi (add immediate) but packed into one instruction.

2. What pc = pc+4+BranchAddr Means on Its Own #

This line describes the core of PC-relative branch addressing, which is how most MIPS branch instructions calculate their target addresses. Let's break each component:

• PC (before the instruction executes): This is the memory address of the rpt instruction itself.
• PC+4: In MIPS, all instructions are 4 bytes long. After fetching an instruction, the PC automatically increments by 4 to point to the next sequential instruction. This value represents the address of the instruction that would run if no branch occurred.
• BranchAddr: This is a signed integer value encoded directly in the rpt instruction's binary format. It's a pre-calculated offset that tells the hardware how far to jump from the "next instruction" (PC+4) to reach the target label (loop).

Putting it all together: pc = pc+4+BranchAddr sets the PC to the address of loop by adding the offset (BranchAddr) to the address of the instruction that would have run next. This is the standard way MIPS handles branches—using a relative offset instead of a full target address saves space in the 32-bit instruction word.

内容的提问来源于stack exchange，提问作者Yinghan Ma