. CSci320 Programming Languages -- laboratory 09 -- C++ Control structures
. Duff's Disgusting Device
Look at this
.See http://groups.google.com/groups?selm=2748%40alice.UUCP

Duff's original code assumes that the place that the data is being moved to is not RAM but
an address of a peripheral!   When run with "to" pointing at RAM it needs a fix.  
I am grateful to an alert reader who corrected me:
.As_is > Given the "8 times", I think you're considering the "*to" (instead
.As_is > of "*to++") being an error.
[As was]
.As_is >
.As_is > May I point out that Tom Duff's application of that routine was to
.As_is > send data to a device register which was memory-mapped at a fixed
.As_is > address, so his code is (for *his* application) correct? This is
.As_is > explained in Duff's original posting[2] as well as (slightly more
.As_is > elaborate) a later comment[3].
.As_is >
.As_is > Of course, the application (memory-to-memory copy vs.
.As_is > copy-to-device-register) is orthogonal to Duff's discussion
.As_is > of language properties.
.As_is >
.As_is > Regards,
.As_is >  Ignatios Souvatzis


Here is a complete working program in C 
.See ../c/duff.c
that uses this device to copy integer arrays.

What changes must you make to move from C to C++?

Notice that it uses a C function header that expects you to provide
the address of two arrays: write a main program that lets you test it.

Publish your working C++ code and link it to a lab page explaining what is
good and bad about Duff's device.

By the way, people
.See http://groups.google.com/groups/search?q=%22duff%27s+device%22&start=0&scoring=d&hl=en&lr=&safe=off&num=10
are still discussing this trick.
. Jackson's Inversion
Here is the design of a co-function that accepts a series of
integers and detects the first and later occurence of the number 2:
	input ::= #(int~2) 2 #(int~2).

.As_is string fun(int i)
.As_is {
.As_is 	restart();
.As_is 	while( i != 2)
.As_is 	{
.As_is 		resume "no two yet";
.As_is 	}
.As_is 	resume "two";
.As_is 	while(true)
.As_is 	{
.As_is 		resume("there was a two");
.As_is 	}
.As_is }
Unfortunately the above uses restart() and resume() which are nonstandard C++ functions.
Resume exits a function but remembers where it was, and restart goes to that place
the `next time the function is call`.  So here is a sample test run
.Table i	returned value
.Row 1	no two yet
.Row 3	no two yet
.Row 2	two
.Row 1	there was a two
.Row 2	there was a two
.Row 3	there was a two
.Close.Table
M. A. Jackson publicised a way to use switch, goto, etc to implement restart and resume.

Here is a file that shows it coded in standard C++.
Download, compile, run and test...
.See ../restart.cpp
Download, compile, and test (99 is the terminating sentinal for input...).
If you have time you can try changing it...

Add to your web page a comment on this style of "inverted code" plus
a link to an example.
. Evil C Macros to make Co-Routines
Co-routines are an alternative way of getting the same effect as Jackson's
inverted code.  Here
.See http://www.chiark.greenend.org.uk/~sgtatham/coroutines.html
is a link into a way to get the same effect in C by using some possible
dangerous macros.  It also has a good discussion of why we need
co-routines.
. Check the Preparation for next class
.See ../10.html

. If you have time
.See http://c2.com/cgi/wiki?ProgrammerLiteracy