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peteraa 2019-10-17 17:39:34 +02:00
parent 90d3bd946f
commit 9da79e3851
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@ -6,11 +6,13 @@
#+begin_src asm #+begin_src asm
addi t0, zero, 10 addi t0, zero, 10
addi t1, zero, 20 addi t1, zero, 20
L2:
sub t1, t1, t0 sub t1, t1, t0
beq t1, zero, .L2 beq t1, zero, .L2
jr ra jr ra
#+end_src #+end_src
** program 2 ** program 2
#+begin_src asm #+begin_src asm
addi t0, zero, 10 addi t0, zero, 10
@ -19,6 +21,7 @@
jr ra jr ra
#+end_src #+end_src
** program 3 ** program 3
#+begin_src asm #+begin_src asm
lw t0, 0(t0) lw t0, 0(t0)
@ -29,7 +32,62 @@
jr ra jr ra
#+end_src #+end_src
* Question 2 - ???
* Question 2 - Handling hazards
For this question, keep in mind that the forwarder does not care if the values it forwards are being used or not!
Even for a JAL instructions which has neither an rs1 or rs2 field, the forwarder must still forward its values.
** Data hazards 1
At some cycle the following instructions can be found in a 5 stage design:
EX: || MEM: || WB:
---------------------||-------------------------||--------------------------
rs1: 4 || rs1: 4 || rs1: 1
rs2: 5 || rs2: 6 || rs2: 2
rd: 6 || rd: 4 || rd: 5
memToReg = false || memToReg = false || memToReg = false
regWrite = true || regWrite = false || regWrite = true
memWrite = false || memWrite = false || memWrite = false
branch = false || branch = true || branch = false
jump = false || jump = false || jump = false
For the operation currently in EX, from where (ID, MEM or WB) should the forwarder get data from for rs1 and rs2?
** Data hazards 2
At some cycle the following instructions can be found in a 5 stage design:
EX: || MEM: || WB:
---------------------||-------------------------||--------------------------
rs1: 1 || rs1: 4 || rs1: 1
rs2: 5 || rs2: 6 || rs2: 0
rd: 0 || rd: 1 || rd: 0
memToReg = false || memToReg = false || memToReg = false
regWrite = true || regWrite = true || regWrite = true
memWrite = false || memWrite = false || memWrite = false
branch = false || branch = true || branch = false
jump = true || jump = true || jump = false
For the operation currently in EX, from where (ID, MEM or WB) should the forwarder get data from for rs1 and rs2?
** Data hazards 3
At some cycle the following instructions can be found in a 5 stage design:
EX: || MEM: || WB:
---------------------||-------------------------||--------------------------
rs1: 2 || rs1: 4 || rs1: 3
rs2: 5 || rs2: 6 || rs2: 4
rd: 1 || rd: 1 || rd: 5
memToReg = false || memToReg = true || memToReg = false
regWrite = false || regWrite = true || regWrite = true
memWrite = true || memWrite = false || memWrite = false
branch = false || branch = false || branch = false
jump = false || jump = false || jump = false
Should the forwarding unit issue a load hazard signal?
(Hint: what are the semantics of the instruction currently in EX stage?)
* Question 3 - Benchmarking * Question 3 - Benchmarking
In order to gauge the performance increase from adding branch predictors it is necessary to do some testing. In order to gauge the performance increase from adding branch predictors it is necessary to do some testing.
Rather than writing a test from scratch it is better to use the tester already in use in the test harness. Rather than writing a test from scratch it is better to use the tester already in use in the test harness.