Specifying circuit properties in PSL

Formal methods

Mathematical and logical methods used in system development

Aim to increase confidence in riktighet of system

Apply to both hardware and software

Formal methods

Complement other analysis methods

Are good at finding bugs

Reduce development (and test) time (Verification time is often 70% of total time in hardware design projects)

Some fundamental facts

Low level of abstraction, Finite state systems

=> automatic proofs possible

High level of abstraction, Fancy data types, general programs

=> automatic proofs IMPOSSIBLE

Two main approaches• Squeeze the problem down into one that can

be handled automatically– industrial success of model checkers– automatic proof-based methods very hot

• Use powerful interactive theorem provers and highly trained staff– for example Harrison’s work at Intel on floating

point algorithms (http://www.cl.cam.ac.uk/users/jrh/)

Model Checking

MC

G(p -> F q)yes

nop

q

p

q

property

finite-state model

algorithm

counterexample

(Ken McMillan)

Again two main approaches• Linear-time Temporal Logic (LTL)

– must properties, safety and liveness– Pnueli, 1977

• Computation Tree Logic (CTL)– branching time, may properties, safety and liveness– Clarke and Emerson, Queille and Sifakis, 1981

Linear time conceptually simplier (words vs trees)

Branching time computationally more efficientWe will return to this in a later lecture

But

temporal logics hard to read and write!

Computation Tree Logic

A sequence beginning with the assertion of signal strt, and containing two not necessarily consecutive assertions of signal get, during which signal kill is not asserted, must be followed by a sequence containing two assertions of signal put before signal end can be asserted

AG~(strt & EX E[~get & ~kill U get & ~kill & EX E[~get & ~kill U get & ~kill & E[~put U end] | E[~put & ~end U (put & ~end & EX E[~put U end])]]])

Basis of PSL was Sugar (IBM, Haifa)

Grew out of CTL (I believe)

Added lots of syntactic sugar

Engineer friendly, used in many projects

Used in the industrial strength MC RuleBase

Assertion Based Verification (ABV) can be done in two ways

During simulation – (dynamic, at runtime, called semi-formal verification,

checks only those runs)

As a static check – (formal verification, covers all possible runs, more

comprehensive, harder to do, restricted to a subset of the property language)

(Note: this duality has been important for PSL’s practical success, but it also complicates the semantics!)

Properties

always (p)

states that p (a boolean expression made from signal names, constants and operators) is true on every cycle

always (! (gr1 & gr2))

Safety Properties

always (p) ”Nothing bad will ever happen”Most common type of property checked

in practiceEasy to check (more later)Disproved by a finite run of the system

Observer: a second approach

Observer written in same language as circuit

Safety properties only

Used in verification of control programs (and in Lava later)

FProp

ok

Back to PSL

always (p) Talks about one cycle at a time

Sequential Extended Regular Expressions (SEREs) allow us to talk about spans of time

A SERE describes a set of tracesIt is a building block for a property

http://www.eda.org/vfv/docs/PSL-v1.1.pdf

SERE examples

{req; busy; grnt}

All sequences of states, or traces, in which req is high on the first cycle, busy on the second, and grnt on the third.

(source Sugar 2.0 presentation from IBM’s Dana Fisman and Cindy Eisner, with thanks)

SERE examples

{req; busy; grnt}

req

busy

grnt

is in the set of traces

SERE examples

{req; busy; grnt}

req

busy

grnt

This too

SERE examples

{req; busy; grnt}

req

busy

grnt

and this

SERE examples

{req; busy; grnt}

req

busy

grnt

but not this

Why?

SERE examples

How can we specify ONLY those traces that start like this?

req

busy

grnt

SERE examples

req

busy

grnt

{req & !busy & !grnt; !req & busy & !grnt; !req & !busy & grnt}

SERE examples

How do we say that the {req,busy,grnt} sequence can start anywhere?

req busy

grnt

SERE examples

{[*]; req; busy; grnt}

req

busy

grnt

[*] means skipzero or more cycles

SERE examples

{[*]; req; busy; grnt}

req

busy

grnt

so our original traceis still in the setdescribed

SERE examples

{true; req; busy; grnt}

req

busy

grnt

says that the req, busy, grntsequence starts exactly in the second cycle. It constrains only cycles 2,3,4

{true[*4]; req; busy; grnt} {true[+]; req; busy; grnt} true[+] =

[+]one or more trues

true[*] = [*]

{[*]; req; busy[*3:5]; grnt}at least 3 and at most 5 busys

{[*]; req; {b1,b2}[*]; grnt}

{[*]; req; {b1,b2,b3}[*7]; grnt}subsequences can also be repeated

&&

Simultaneous subsequencesSame length, start and end together

{start; a[*]; end} && {!abort[*]}

|

One of the subsequences should be matchedDon’t need to be the same length

{request; {rd; !cncl_r; !dne[*]} | {wr;!cncl_w;!dne[*]};dne}

Fancier properties at last!

SEREs are themselves properties (in the latest version of PSL). Properties are also built from subproperties.

{SERE1} |=> {SERE2} is a property

If a trace matches SERE1, then itscontinuation should match SERE2

if then

{true[*]; req; ack} |=> {start; busy[*]; end}

Not just the first req, ack{true[*]; req; ack} => {start; busy[*]; end}

if then

if then

Overlap also possible!{true[*]; req; ack} => {start; busy[*]; end}

if then

ifthen

if then

{true[*]; req; ack} => {start; data[*]; end}

{true[*]; req; ack} => {start; data[=8]; end}

if then

1 2 3 4 5 6 7 8

Can check for data in non-consecutive cycles

A form of implication

{SERE1} |=> {SERE2}If a trace matches SERE1, then itscontinuation should match SERE2

Another form of implication

{SERE1} |-> {SERE2}If a trace matches SERE1, then SERE2

should be matched, starting from the last element of the trace matching SERE1

So there is one cycle of overlap in the middle

Example

{[*]; start; busy[*]; end} |-> {success; done}

If signal start is asserted, signal end is asserted at the next cycle or later, and in the meantime signal busy holds, then success is asserted at the same time as end is, and in the next cycle done is asserted

Example

{[*]; {start; c[*]; end}&&{!abort[*]}} |-> {success}

If there is no abort during {start,c[*],end}, success will be asserted with end

{SERE1} |=> {SERE2} = {SERE1} |-> {true, SERE2}

Both are formulas of the linear fragment(which is based on LTL)In Jasper Gold, we use this linear part.

There is also an optional branching extension (which is where CTL comes back in)

PSL has a small core and the rest is syntactic sugar, for example

b[=i] = {not b[*]; b}[*i] ; not b[*]

See formal semantics in LRM

PSL

Regular expressions (plus some operators)+Linear temporal logic (LTL)+ Lots of syntactic sugar+ (optional)Computation tree logic (CTL)

Example revisited

A sequence beginning with the assertion of signal strt, and containing two not necessarily consecutive assertions of signal get, during which signal kill is not asserted, must be followed by a sequence containing two assertions of signal put before signal end can be asserted

AG~(strt & EX E[~get & ~kill U get & ~kill & EX E[~get & ~kill U get & ~kill & E[~put U end] | E[~put & ~end U (put & ~end & EX E[~put U end])]]])

In PSL (with 8 for 2)

A sequence beginning with the assertion of signal strt, and containing eight not necessarily consecutive assertions of signal get, during which signal kill is not asserted, must be followed by a sequence containing eight assertions of signal put before signal end can be asserted

always({strt; {get[=8]}&&{kill[=0]}}

|=> {{put[=8]}&&{end[=0]}})

PSL

Seems to be reasonably simple, elegant and concise!

Jasper’s Göteborg based team have helped to define and simplify the formal semantics.

See the LRM and also the paper in FMCAD 2004

Friday’s lecture

About Jiri Gaisler’s two process method of using VHDL

Next week, I will return to CTL and how to model check it

Note, I will omit LTL model checking in this year’s course