Proposition of Reservation Station implementation

Here is an example proposition how to implement RS using transaction framework for internal communication. If you want you can follow this proposition, if you don’t, feel free to change anything you want.

Internal data

Actual Reservation Station

This is a buffer which has R rows. Each row has the following structure:

v	opcode	`id_out`	`id_ROB`	`id_rs1`	`val_rs1`	`id_rs2`	`val_rs2`

v - “valid” - it is 1 if entry is a correct instruction which waits to be filled with operands/dispatched
id_rsX - is 0 when the source value is ready (and is stored in the appropriate val_rsX) or not needed. It is non-zero when we wait for an operand to be ready.
When the operand is ready we insert it to the appropriate val_rsX field and we put zero to id_rsX
The instruction is ready to be dispatched if v is 1 and both id_rs1, id_rs2 are 0

Used slots table

It is a table with R one-bit fields. Each field is 1 if this slot is used or is reserved to be used in the near future (there is instruction in pipeline which will be saved to this slot).

When an entry in the RS is released, the corresponding entry in this table is switched from 1 to 0.

Internal methods

Compare and substitute

Input:

tag - identifier of field in RF which should be compared with id_rsX to check if there is need to store value
value - value which should be stored in RS field if tag matches
position - position in RS on which comparison and replacement should take place

Output:

null

Side effects:

When tag matches one of id_rsX saved in RS on position then id_rsX is cleared (set to 0) and value is saved in val_rsX

Read row

Input:

position - which RS row should be read

Output:

opcode - instruction opcode for FU
val_rs1 - value of first operand
val_rs2 - value of second operand
id_out - id of RF field where instruction output should be stored
id_ROB - id of ROB entry which is allocated for this instruction

Side effects:

null

Clean row

Input:

position - RS row which should be cleaned

Output:

null

Side effects:

v bit for entry on position set to 0

Get free slot

Input:

null

Output:

position - of the free slot in the RS

Side effects:

null

Mark slot as used

Input:

position - of slot which should be marked as used

Output:

null

Side effects:

position slot in RS marked as used

Proposition of implementation of external interfaces

Get slot and mark as used

“Get free slot” position and call “Mark slot as used” with it.

Compare and substitute all

Invokes “Compare and substitute” for each row of RS.

Read and clean row

For given row r atomically “Read row” and “Clean row”