Letters

Dear Editor:

To answer partially Joe Sutton’s question (SYNC 1 :4) about the 8K ROM’s “double image,” I noticed the same thing with the 4K ROM: 4 “images” of the same 4K monitor program “were to be found in the 16K addressing space set aside for system ROM. I reasoned that the unneeded most significant bits of the address bus were ignored, enabling the ROM chip whenever the address was below 16K. Thus, for the 8K ROM, address and address plus 8K are interpreted as the same address. The 8K bit is ignored. However, I do not understand the nitty-gritty hardware aspects fully (the ROM appears to have both the A8 and A12 address lines connected to chip select).

I would like to see an indepth discussion of the ZX80’s discrete components (not much to discuss in the ZX81!). I would also like to see more articles on the 4K and 8K ROMs as well as hardware interfaces. For instance, I would like to purchase a surplus terminal (Model 15 or Model 33), but I do not know whether the standard Z80 interface will suffice or what modifications of the 8K ROM’s LPRINT are needed.

As a math teacher I would like to see articles on educational applications since with tight money in government the inexpensive, but effective ZX81 may be the only realistic choice for today’s schools.

I really have to compliment Mr. Sinclair on the improvements and reduced price of the ZX81. I only wish I would have purchased a ZX81 kit rather than an 8K ROM upgrade; then I would have two computers and smooth display for only $60 more. It would also be nice to have more on board RAM. So how about an article on constructing a memory upgrade using one of the many available RAM PC boards?

I would also like to see an article on constructing or interfacing a 2716/2732 EPROM programmer (student proof programs!

Timothy Mcllwee
Granada Royale Rm. 121
Route 1 and I-95
Ormond Beach, FL 32074

Ed. —Here is a list of challenges for SYNC authors. Similar requests have come from other readers.

Dear Editor:

The program Taxman that appears in SYNC ( 1 :6) is an interesting mathematical game. The listing (as printed) does contain one minor bug. Line 630 should read

630 FOR 1=3 TO N to cover all possible cases. It would not hurt if, in fact, that line were to read

630 FOR 1 = 1 TO N
As presently written, line 630 will not allow the computer to have credit for a remaining unused value of 3. To test this out, run the program for a set of 5 numbers.

It is not difficult to show that for a set of 30 numbers, the best score attainable is 301 to 164. The opening choices are 29, 25, 1 5, and 27. The remainder of the solution is left as an exercise for the reader. The first move in this game is obviously to pick the largest prime number in the set.

Peter D. Hoffman
5618 Martinique Dr.
Corpus Christi, TX 78411

Dear Editor:

In reference to Dr. Stephen A. Justham’s article entitled “An Inventory System,” much space and some computing time can be saved in the search sequence by making the following changes:

3 DIM R$(1,15) 
3040 INPUT R$(1) 
3042 delete 
3044 delete 
3052 delete
3068 IF R$( 1)=I$(B) THEN GOTO 3100 
3115 PRINT TAB 5; R$(1)

By setting up a single position array of 15 length at line 3, the input at line 3040 is automatically adjusted to equal the elements of the I$ array. This saves setting up the 151 position M$ array, resulting in a saving of up to 2,250 bytes of RAM.

Ralph Goodrick
3700 W. 151st
Stanley, KS 66224

Dear Editor:

I feel “Hampson’s Plane” is one of the better 4K/1 K games you have published. I made a minor change to make continuous play easier. Add lines:

1 CLS 
105 IF K$="X" THEN GOTO 1

When one has completed a plane (or whenever an alpha coordinate is requested) an “X” can be entered to return to the beginning of the game. I also suggest that the string in line 20 be changed to one space. It makes the appearance of a plane similar to the one published in the article.

Robert Masters
396 Billerica Road
Tewksbury, MA 01876

Dear Editor:

I wish to pass on a tip to readers of SYNC concerning flags. In games involving two players (and in other two-way situations), flags are often used to determine which group of data is to be used. The actual setting and resetting of flags would usually go something like this:

10 LET  A=0
100 LET A=1 (or 100 LET A=A+1).

Suppose that you don’t know if the flag is set, but you want to change its state. You have to type:

100 IF A=1 THEN GOTO 115 
105 LET A=1 
110 GOTO 120 
115 LET A=0

A much easier way is to type

100 LET A=ABS(A-1)

This does the exact same thing with much less memory space.

Philip Gervais
714 5th Ave.
So. Clinton, IA 52732

Dear Editor:

Wow! Am I ever impressed. I had no idea of the breadth of the ZX 80/81 world. As I have had my ZX 80 only two months, some things in the programming completely eluded me. My very first issue of SYNC (1:5) really opened up a lot of grey areas for me.

Anyhow, the main reason for this letter is the availability of the first 4 issues. If you have them on hand, please mail and bill. If not, do you know of an outlet for same.

I will appreciate any assistance.

Tony Wall

Ed— At this point we have only a few copies of 1:2 and 1:4. Back issue orders must include payment of $2.50 per magazine. There are no other outlets.

Dear Editor:

I enjoyed Chuck Dawson’s “The Great Circle Route” (SYNC 1:5) and found it a good exercise to use it in conjunction with a standard mileage chart which gives distances in nautical miles. However, I felt that the addition of city names and approximate cost would make an interesting improvement. The cost figures are based on the average cost of Transcontinental U.S. flights and can be changed by adjusting the .009 figure in line 205.

A report code 5 (FULL SCREEN) will appear after destination LATITUDE prompt and it will be necessary to use CONTINUE.

The program changes are:

20 PRINT "ENTER DEPARTURE CITY"
21 INPUT A$ 
23 PRINT A$
100 PRINT "DESTINATION CITY"
101 INPUT B$ 
103 PRINT B$
185 PRINT A$;,B$
205 PRINT "COST#S";.09*DIST;"####"

H.S. Wake
4171 Stealer Way
San Diego, CA 92122