Small step semantics help needed

[MorF]

Hello,
due to my sickness I was absent on the lecture 9 - Code Generation and
now I feel really confused about the "small step semantics".

I simply don't understand the syntax and what does it mean?
for e.g goto instruction is explained with this sentence

<goto LABEL, i-C-V-S> --> <C(LABEL), LABEL-C-V-S>

but then (solutions to exam 2008-2
http://www.cse.chalmers.se/edu/course/TIN321/exams/solutions-plt-2008-2.html
)
there is something:
<goto L, C-P-V-S> ---> < C(L), C-L-V-S>

What does C-L-V-S or C-P-V-S stand for?

I'd be very thankful for any help!

[Aarne Ranta]

Hello MorF,

The explanation is in lecture 8, interpreters. To summarize, your example means:

  execution of instruction goto LABEL, in the state consisting of

  - current instruction pointer i

  - code C (i.e. all instructions)

  - variable storage V

  - stack S

  leads to the state with

  - next instruction C(LABEL) (i.e. the instruction in C pointed by LABEL)
  - current instruction pointer LABEL

  - code C (not changed)

  - variable storage V (not changed)

  - stack S (not changed)

Thus the rules of the form <...>--> <...> tell how the state is changing. They can be seen as compact formulations of the textual explanations in the JVM specification, see

  http://java.sun.com/docs/books/jvms/second_edition/html/Instructions2.doc.html

(also referenced from exercise 1). Now you can go through all small-step rules in lectures 8 and 9 and verify that this is the case; alternatively, you can take it as an exercise to formulate some JVM specs as semantic rules and compare with the lectures. (However, for goto this looks a little different, because the JVM spec is closer to the bytecode than the Jasmin assembler, which we use ; start with iadd for a more exact correspondence.)

Regards

  Aarne.

[MorF]

Thank you very much for the explanation!
I think now I understand it :-) Seemed much more complicated than it
actually is.

On Mar 13, 1:09 pm, Aarne Ranta <aa...@chalmers.se> wrote:
> Hello MorF,
>
> The explanation is in lecture 8, interpreters. To summarize, your example
> means:
>
>   execution of instruction goto LABEL, in the state consisting of
>   - current instruction pointer i
>   - code C (i.e. all instructions)
>   - variable storage V
>   - stack S
>   leads to the state with
>   - next instruction C(LABEL) (i.e. the instruction in C pointed by LABEL)
>   - current instruction pointer LABEL
>   - code C (not changed)
>   - variable storage V (not changed)
>   - stack S (not changed)
>
> Thus the rules of the form <...>--> <...> tell how the state is changing.
> They can be seen as compact formulations of the textual explanations in the
> JVM specification, see
>

> http://java.sun.com/docs/books/jvms/second_edition/html/Instructions2...

>
> (also referenced from exercise 1). Now you can go through all small-step
> rules in lectures 8 and 9 and verify that this is the case; alternatively,
> you can take it as an exercise to formulate some JVM specs as semantic rules
> and compare with the lectures. (However, for goto this looks a little
> different, because the JVM spec is closer to the bytecode than the Jasmin
> assembler, which we use ; start with iadd for a more exact correspondence.)
>
> Regards
>
>   Aarne.
>

> On Sun, Mar 13, 2011 at 12:59 AM, MorF <nuzz...@gmail.com> wrote:
> > Hello,
> > due to my sickness I was absent on the lecture 9 - Code Generation and
> > now I feel really confused about the "small step semantics".
>
> > I simply don't understand the syntax and what does it mean?
> > for e.g goto instruction is explained with this sentence
>
> >    <goto LABEL, i-C-V-S>  -->  <C(LABEL), LABEL-C-V-S>
>
> > but then (solutions to exam 2008-2
>

> >http://www.cse.chalmers.se/edu/course/TIN321/exams/solutions-plt-2008...