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The simplified DLX. The datapath & control story: What happens in each control state. The Datapath. The Datapath consists of several environments, buses, registers and multiplexers. At the right there is a general scheme of the entire Datapath (no drivers, only muxes). datapath.pdf. - PowerPoint PPT Presentation
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The simplified DLX
The datapath & control story:
What happens in each control state.
The Datapath
The Datapath consists of several environments,
buses, registers and multiplexers .
At the right there is a general scheme of the
entire Datapath (no drivers, only muxes).
datapath.pdf
What is RTL?
RTL = Register Transfer Language
The RTL describes the operations done in each Control’s state in terms of “what is written to the registers.”
Note: This description is just of the functionality and not of the way of implementation.
Examples:
• IR=M[PC]• A=RS1• B=RS2• PC=PC+1
The ControlThe Control is a state machine that consists of 20 states.
The Control’s signals control the data flow .
fsd_idlx.pdf
What happens during Fetch state?
DinAdr
Dout
W
Busy
MemoryEnv.
Control
IR Env.
IR
PC
MR Was the mission
completed?
Sample!1) The PC register contains the address.
2) The desired functionality: IR M[PC]
FetchThe Control usually stays in fetch state
for more than one clock cycle
3) The memory is very slow. It announces: “I’m busy” after getting a read / write request. When busy, no new requests are allowed. As Dout stablizes, the memory announces: “I’m done”. Only then the control asks the IR Env. to sample Dout’s value and update the IR register.
Reminder…
In our explanation about the different states we also describe the RTL instructions that correspond to each state of the Control. Let’s start:
1 (Fetch .
RTL Instruction: IR=M[PC]
Action:The instruction pointed to
by the PC is read from the memory and
is stored to the IR register.
The Fetch state might last more than one clock cycle. The DLX stays in this state as long the “busy” signal is active.
The Control (Cont.)
Active Control Signals: IRce, PCAOdoe, MR
IR’s Clock Enable
The PC Env. address out’s
driver is enabled
Memory Read
The Control (Cont.)2 (Decode .
RTL Instructions: A=RS1
B=RS2PC=PC+1
Actions:The contents of the
RS1 and RS2 (if needed) registers are stored in the A and B registers
) that are located in the GPR Env (.and the PC’s register value
is incremented by 1 )to point to the next instruction.(
Active Control Signals: Ace, Bce, PCS1doe, 1S2doe, ALUDINTdoe, Add, PCce
PC’s value’s driver is enabled and the PC is outputted to S1
1’s const value’s driver is enabled and this value is outputted to S2
The ALU’s output value’s (PC+1) driver is enabled
PC’s clock enable (for storing the new PC)
GPR Env.
A B
C
Cce IR Env.
1 0 ITYPE
ALU Env. ALU Control signals
Result
A. Arithmetic/Logic instructions:
What happens during Execute state (Cont.)?
Sext (Imm.)
Reminder…
3 (ALU .
RTL Instruction:C=A op B
Action:An arithmetic operation is done
on the arguments (which are the contents of the A and B registers.(
The result is stored in the C register in the GPR Env.
Active Control Signals: AS1doe, BS2doe, OPALU, ALUDINTdoe, Cce
The Control (Cont.)
The name of a group of signalsThe A register’s value’s
driver is enabled and its value is outputted to S1
The B register’s value’s driver is enabled and its value is outputted to S2
The Control (Cont.)
4 (ALUI (Add) .
RTL Instruction:C=A + imm.
Action:The sum of the addition of
the value of the A register in the GPREnv. and the immediate’s value is
stored in the C register in the GPR Env.
Active Control Signals: AS1doe, IRS2doe, OPALU, ALUDINTdoe, Cce
The IR register’s value’s driver is
enabled and its value (=immediate) is outputted to S2
The Control (Cont.)
5 (ShiftI .
RTL Instruction:C=ShiftLeft A / ShiftRight A
Action:The value of the A register is being
shifted one step right or left .The direction is determined by
IR[1].
Active Control Signals: AS1doe, Cce, shift (left or right).
In a general shifter: the shift amount is coded in IR…
B. Test&Set instructions:
Sgri: RD (RS1 > Sext(Imm) ? 1 : 0)
GPR Env.
A B
C
Cce IR Env.
1 0 ITYPE
ALU Env. TEST
Result
Sext (Imm.)
Two stages:1) executeC (RS1 > RS2 ? 1:0)
2 (writebackRD CIn this way the period
time is shorter .
What happens during Execute state (Cont.)?
Always 0 / 1
Reminder…
6 (TestI
RTL Instruction: C=A rel imm.
Action:A logic operation is done
on the arguments )which are the contents of
the A register and the immediate valuethat is supplied by co in the IR Env.(.
The result is stored in the C register in the GPR Env.
Active Control Signals: AS1doe, IRS2doe, Cce, ALUDINTdoe, test
Comparison to be conducted: can be extracted from last 3 bits in the opcode.
The Control (Cont.)
3) Jump instructions:
A. Unconditional jump: Jump Reg (jr): PC A
What happens during Execute state (Cont.)?
No direct path from A to PC. Instead:
GPR Env.
AIR Env.
0
ALU Env. ADD
PC Env.
PC Cce
Reminder…
8 (JR .
RTL Instruction:PC=A
Action:In this state that deals with
jumping, the PCregister gets the address of the
instruction to jump to. This addressis stored in the A register in the GPR
Env.
Active Control Signals: AS1doe, 0S2doe, add, ALUDINTdoe, PCce
The Control (Cont.)
C. Calling a routine: ”Jump, remember your address so it will be possible to get back to this address”
jalr: R31 PC+1
PC RS1
We use two Control states in order to execute this instruction because we want to avoid a collision in the buses (to be elaborated).
What happens during Execute state (Cont.)?
Reminder…
State I: Copying the PC (2 clock cycles)
GPR Env.
C
PC Env.
PC Cce
Clock cycle 1 (savePC):
GPR Env.
C
R31
Clock cycle 2 (like in the Write-Back state):
Write!
The address=31
What happens during Execute state (Cont.)?
Reminder…
State II (JALR): Calculating the jump address
PC Env.
PC
ALU Env.
IR Env.0
GPR Env.
A
ADD
Cce
What happens during Execute state (Cont.)?
Reminder…
The Control (Cont.)
9 (SavePC .
RTL Instruction:C=PC
Action:The PC is stored
in the C register in the GPR Env.
Active Control Signals: PCS1doe, 0S2doe, add, ALUDINTdoe, Cce
The Control (Cont.)
17 (JALR.
RTL Instructions:PC=A
R31=C (=old PC)
Actions:In this state that deals with jumping,
the PC’s current value is stored in the 31’st register in the GPR
and the address of the instruction we jump to is stored in the PC and
comes from the A register in the GPR.
Active Control Signals:AS1doe, 0S2doe, ALUDINTdoe, add, PCce, GPR_WE, Jlink
B. Branch: “Jump only if a condition is satisfied”
beqz: PC PC+1+(RS1=0 ? Sext (Imm.) : 0)
bnez: PC PC+1+(RS1=0 ? 0 : Sext (Imm.))
We’ll demonstrate the beqz instruction. Two states are needed:
State I - Branch: Check the condition – RS1=0 ? Sext (Imm.) : 0
GPR Env.
AIR Env.
0
ALU Env. TEST
Comparison result
To the Control
What happens during Execute state (Cont.)?
Reminder…
10 (Branch .
RTL Instruction:Branch taken?
Action:In the Beqz and Bnez instructions ,
when reaching this state, a check ofthe A register’s value is done.
According to this check, a path to continue with is determined.
The Control (Cont.)
Active Control Signals: AS1doe, 0S2doe, test
The next state
BTakenFetch
State II - BTaken: Calculating the jump – PC PC+1+(Result of state I)
IR Env.
ALU Env.
PC Env.
PC Cce
PCSext (Imm.)
ADD
What happens during Execute state (Cont.)?
JumpNo
Jum
p
Reminder…
18 (BTaken .
RTL Instruction:PC=PC+imm.
Action:The PC gets the address of
the next instruction to be executed.The address is based on the Branchstate’s result. The value to be added
to the PC comes from the cooutput of the IR Env.
Active Control Signals: PCS1doe, IRS2doe, add, ALUDINTdoe, PCce
The Control (Cont.)
The Control (Cont.)
11 (WBR .
RTL Instruction:RD=C (R-type)
Action:The data which is stored in the Cregister in the GPR env. (after thearithmetic\shift operations are done(
is being assigned to the instruction’starget register .
Active Control Signal: GPR_WE
12 (WBI .
RTL Instruction:RD=C (I-type)
Action:Copy the result stored in the C
register in the GPR env. (after thearithmetic\logic operations are done(
to the instruction’starget register .
Active Control Signals: GPR_WE, ITYPE
The Control (Cont.)
Reading From Memory - Load
Load Word (lw): RD M(Sext(imm.) + RS1)
Four states are needed for finishing the load instruction:
State I: Effective Address Computation – MAR A+C0
GPR Env.
A
IR Env.
ALU Env.
MAR
C0
ADD
Sext(imm.) + RS1
Sext(imm.)
Reminder…
The Control (Cont.)
7 (AddressCmp .
RTL Instruction:MAR=A+imm.
Action:The memory address where a
data should be written to is stored in the MAR register. The address itselfis the sum of the immediate valuesupplied by the IR Env. and the A
register in the GPR Env.
Active Control Signals: AS1doe, IRS2doe, add, ALUDINTdoe, MARce.
State II: Memory Access (Load). This state lasts till the value is returned – MDR M(MAR)
Reading From Memory – Load (Cont.)
Adr
Dout W
MemoryEnv.
MAR
Read!
MDR
Notes:
1) Busy signal informs the Control when the operation is over (Dout is stable).
2) MDR samples Dout every clock cycle, no need to compute CE (Simplifies Control).
3) Dout must be logical even if its value is incorrect!
Busy
Tells the Control when readung is over
Reminder…
The Control (Cont.)
13 (Load .
RTL Instruction:MDR=M[MAR]
Action:Store M[MAR] in MDR
Active Control Signals: MR, MDRce, Asel, MDRsel
State III (copy MDR to C): Writing MDR’s value to C in the GPR : C MDR
Reading From Memory – Load (Cont.)
MDR
GPR Env.
C
Cce
State IV: Write-Back: RD C
Reminder…
16 (CopyMDR2C.
RTL Instruction:C=MDR
Action:Copy MDR contents to C register in the GPR.
The Control (Cont.)
Active Control Signals: MDRS1doe, 0S2doe, add, ALUDINTdoe, Cce.
GPR Env.
B
Writing To Memory – Store (Cont.)
MDR
State II: Copying the B register’s (this is RD) value to the MDR : MDR B
State III: Memory Access (Store) – M(MAR) MDR
Adr
W
MemoryEnv. MAR
Write!
MDRDin
Busy
Tells the Control when writing is over
Reminder…
The Control (Cont.)
15 (CopyGPR2MDR .
RTL Instruction:MDR=B
Action:The value to be next written to
the memory is stored in this statein the MDR. The value itself comes
from the B register in the GPR.
Active Control Signals: MDRS1doe, BS2doe
14 (Store .
RTL Instruction:M(MAR)=MDR
Action:The instruction that is stored in theMDR is copied to the memory Env.The address in the memory Env.
is taken from the MAR.
Active Control Signals: MARS1doe, MW
The Control (Cont.)
