RS_impl

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"