Before you can read the Reservation stations parent note, you must own every word it throws at you. This page builds each symbol from nothing — plain meaning, a picture, and why the topic needs it. Read top to bottom; each block leans on the one above.
Picture a tiny recipe card: the top says what to do (add), the middle lists ingredients (operands), the bottom is the finished dish (result).
The topic needs these because a reservation station's whole job is: hold the recipe, wait for missing ingredients, deliver the dish. If you can't name the recipe card's three parts, nothing later makes sense.
Look at the figure: eight labelled cubbyholes. R2 currently holds the number 7, R3 holds 4. An instruction like R1 = R2 + R3reads cubbyholes R2 and R3 and writes cubbyhole R1.
Ready: its number already exists. We can copy it into the desk right now.
Pending: some earlier, not-yet-finished instruction will produce it. We must wait.
Why the topic needs this split: a station can't execute while any operand is a sticky note. The moment the last sticky note becomes a jar, it fires. Everything else is bookkeeping around this one condition.
Why j and k? Because most operations (ADD, SUB, MUL) take exactly two inputs. j is the first, k is the second. A load needs only one; then k is simply unused.
Look at the figure: one speaker (the finishing unit) shouts a (tag, value) pair onto a single wire that touches every desk and the register boxes. Each desk compares the shouted tag against its own Qj and Qk. On a match, it replaces the sticky note with the jar.
Follow the figure left to right. Initially R1's row is empty → its value is trustworthy. When MUL R1,… issues at desk Mult1, we stamp Status[R1] = Mult1: "don't trust the box; Mult1 owns R1's future." Later SUB also targets R1 at desk Add2, so we overwrite to Status[R1] = Add2.
Every arrow says "you need the left before the right makes sense." The bottom node Z is the parent topic: an instruction executing the instant its jars are full.
What is the difference between a register's name and its value?
The name is the label on the box; the value is the number inside. Renaming re-points which desk owns a name without touching stored numbers.
When do we use Vj versus Qj?
Vj holds the operand value when it is ready (jar on desk); Qj holds the tag of the producing desk when the value is still pending (sticky note).
What does Qj=0 mean?
No tag pending — the operand's value has arrived. It does NOT mean the value equals zero.
What is a tag?
A short ID naming one specific reservation station, used as the return address on a sticky note so consumers know whose result to grab.
Why broadcast on a CDB instead of point-to-point wires?
Point-to-point needs about n2 wires; one shared shout-wire reaches all n consumers with O(n) wiring, and we don't know at issue time who will need the result.
How many bits does a tag need for 64 stations?
⌈log264⌉=6 bits.
Which hazard is "true" and which two are "false"?
RAW is true (you genuinely lack the data); WAR and WAW are false, caused only by reusing the same register name — renaming removes them.
What does overwriting Status[R1] accomplish?
It makes the newest writer the owner of R1, so later readers depend on the latest value and earlier dead writes become invisible (WAW/WAR fix).
