A one-issue processor uses out-of-order execution with a Reorder Buffer (ROB). The processor has two unified reservation stations (RS0 and RS1) and four ROB entries (ROB0 through ROB3). There are two fully pipelined execution units. One unit only executes divide (DIV) instructions and has a 5-cycle latency. The other unit executes all other instructions and has a 2-cycle latency. All units need their full latency to compute the result (there is no left-over time in the last cycle of execution). Only one result can be broadcast in each cycle. A reservation station that has been used by an instruction is freed when the instruction is selected for execution, and another instruction can be issued into that RS during the first execution cycle of the original instruction. A ROB entry can be reused in the cycle after the one in which it was freed (you should know when that is). An instruction that is waiting for a result to be produced can begin execution in the cycle after the one in which its last missing operand is broadcast.

Assume that all reservation stations and ROB entries are empty at the beginning of the starting cycle (Cycle 1), and that the processor has already fetched, decoded, and perfectly branch-predicted the following instructions, so they are all waiting to be issued at the beginning of the starting cycle (Cycle 1). We have filled out the first row of the execution timing table below. You need to fill the rest of it.

Note: The points in this question are awarded depending on the demonstrated application of concepts and rules in instruction scheduling, not based on the number of "boxes" that contain the correct numbers. In other words, the boxes only exist to enter your overall answer to problem in a manageable way, but the points are awarded for the problem as a whole, not for each individual answer box.

Question

A one-issue processor uses out-of-order execution with a Reorder Buffer (ROB). The processor has two unified reservation stations (RS0 and RS1) and four ROB entries (ROB0 through ROB3). There are two fully pipelined execution units. One unit only executes divide (DIV) instructions and has a 5-cycle latency. The other unit executes all other instructions and has a 2-cycle latency. All units need their full latency to compute the result (there is no left-over time in the last cycle of execution). Only one result can be broadcast in each cycle. A reservation station that has been used by an instruction is freed when the instruction is selected for execution, and another instruction can be issued into that RS during the first execution cycle of the original instruction. A ROB entry can be reused in the cycle after the one in which it was freed (you should know when that is). An instruction that is waiting for a result to be produced can begin execution in the cycle after the one in which its last missing operand is broadcast.

Assume that all reservation stations and ROB entries are empty at the beginning of the starting cycle (Cycle 1), and that the processor has already fetched, decoded, and perfectly branch-predicted the following instructions, so they are all waiting to be issued at the beginning of the starting cycle (Cycle 1). We have filled out the first row of the execution timing table below. You need to fill the rest of it.

Note: The points in this question are awarded depending on the demonstrated application of concepts and rules in instruction scheduling, not based on the number of "boxes" that contain the correct numbers. In other words, the boxes only exist to enter your overall answer to problem in a manageable way, but the points are awarded for the problem as a whole, not for each individual answer box.

BlackTom AI · Accepted Answer

The provided data payload contains a full problem statement but there are no answer options to analyze, and the answer field is empty. To guide understanding, I will restate what is given and outline how to approach filling in the timing table, along with common reasoning steps you would apply once options or a concrete answer grid are available.

Restatement of what is given:
- A one-issue processor uses out-of-order execution with a Reorder Buffer (ROB).
- There are two unified reservation stations (RS0 and RS1) and four ROB entries (ROB0–ROB3).
- There are two fully pipelined execution units:
  • One unit executes only DIV instructions and has a 5-cycle latency.
  • The other unit executes all other instructions and has a 2-cycle latency.
- Each unit requires its full latency to compute the result; there is no leftover time in the last cycle of execution.
- Only one result can be broadcast in each cycle.
- A reservation station used by an instruction is freed when the instruction is selected for execution, and another instruction can be issued into that RS during the first execution cycle of the original instruction.
- A ROB entry can be reused in the cycle after the one in which it was freed.
- An instruction waiting for a result can begin execution in the cycle after the last missing operand is broadcast.
- At Cycle 1, all RS and ROB entries are empty. The processor has already fetched, decoded, and perfectly branch-predicted the following instructions, so they are waiting to be issued at the beginning of Cycle 1. The first row of the execution timing table has been filled; you must fill the rest.
- The evaluation emphasizes correct application of scheduling rules, not the number of boxes filled.

What is missing to proceed:
- The actual sequence of instructions (types and operands) that follow in the program after Cycle 1. The problem text describes that they are waiting to be issued, but does not list them here.
- The exact timing table structure (i.e., how many cycles are expected, which columns correspond to which instruction, and what information each box should contain). Without the specific table layout or the instruction sequence, we cannot numerically populate the remaining cells.
- Any constraints or notes about data dependencies between the upcoming instructions beyond what is described (e.g., whether there are implicit multiply/divide dependencies, or any instructions that always use the same resources).

How you would proceed once you have the full data (i.e., the instruction sequence and the timing table structure):
1) Map each instruction to resource requirements and latency:
   - If the instruction is DIV, assign it to the DIV execution unit with a 5-cycle latency.
   - If the instruction is any other type, assign it to the other execution unit with a 2-cycle latency.
2) Track ROB and RS usage cycle by cycle:
   - When an instruction is issued into an RS, mark that RS as occupied until the instruction is selected for execution in that cycle, after which the RS is freed for a new instruction; note that a new instruction can be issued into the freed RS in the first execution cycle of the original instruction.
   - When an instruction enters the ROB, reserve a ROB entry until its result is broadcast; the ROB entry can be reused in the cycle after it was freed. You must account for the rule that an ROB can be reused the cycle after freeing.
3) Determine issue and execution timing:
   - An instruction can begin execution in the cycle after all its operands are available (i.e., after the last missing operand is broadcast).
   - Each instruction’s execution must run for its full latency; no tail-time savings apply.
   - Only one result can be broadcast per cycle, so writeback/broadcast becomes a potential bottleneck that may stall issue of dependent instructions or scheduling.
4) Fill the timing table row by row:
   - For each issuing instruction, record the cycle of issue, the RS/ROB allocations, the start of execution, the cycles of its execution, and the broadcast cycle of its result.
   - Update resource availability accordingly, freeing RS and ROB entries precisely as per the stated rules.
5) Validate with potential edge cases:
   - If multiple instructions become ready in the same cycle, choose an ordering that obeys the one-result-per-cycle constraint for broadcast and the single-issue-per-cycle rule.
   - If a DIV instruction is ready but the DIV unit is busy, the instruction may wait in the RS or ROB until the DIV unit becomes available, depending on the exact issue policy described (the problem statement emphasizes availability and full-latency execution but may permit certain forwarding behaviors).

Potential misconceptions to avoid:
- Confusing the cycle in which an instruction is freed from its RS with the cycle in which a new instruction can be issued into that RS. The rule states a new instruction can be issued into that RS during the first execution cycle of the original instruction, which requires careful synchronization.
- Assuming partial execution time saves some latency. The problem explicitly states that there is no leftover time in the last cycle of execution for any unit.
- Ignoring the one-broadcast-per-cycle constraint as a strict limiter on when results can be observed by dependent instructions.

In summary, with the actual instruction sequence and the exact table format, you would step through cycles 1 onward, assigning each instruction to RSs and ROB entries, scheduling their execution according to latency, freeing resources at the specified times, and recording the broadcast cycles for their results. If you can provide the missing instruction sequence and the exact table structure (or the answer options for this question), I can walk through a concrete, cycle-by-cycle fill of the remaining table entries and explain each choice in context.

CS-6290-O01 Midterm

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