Pete's QBASIC Site

I come back on my program:

5 CLS

10 DIM A$(4, 4)
20 DATA A,B,C,D
30 DATA E,F,G,H
40 DATA I,J,K,L
50 DATA M,N,O,P
55 FOR I = 1 TO 4
58 FOR J = 1 TO 4
60 READ A$(I, J)
90 NEXT J: NEXT I: FOR I = 1 TO 4
100 PRINT A$(I, 1), A$(I, 2), A$(I, 3), A$(I, 4)
110 NEXT I
115 PRINT
120 DEF SEG = VARSEG(A$(1, 2))
125 POKE VARPTR(A$(1, 2)), (90)
130 FOR L = 1 TO 4
140 PRINT A$(L, 1), A$(L, 2), A$(L, 3), A$(L, 4)
150 NEXT L

It would work in QB7.1 and if somebody is interested, he can copy it and paste it directly in QB7.1 ,the lines numbers do not matter.
What I get is the original table ,except the element A$(1,2),in spite of instruction 125 (POKE etc)
Can somebody explains why the poked value do not writes itself inside the table at the place of the previous "B"???

In QB71:
When using strings, use: SSEG and SADD instead of VARSEG and VARPTR

And you dont need line numbers in QB

Great! The reason why there are line numbers is because the program should be written in GW BASIC. It is really amazing to see that QB 7.1 accepts line numbers as I discovered it!
Now may I again ask something? I would like to find litterature on offset address etc to explain how to play with this:
ACH=PEEK(AD+1)*256+PEEK(AD)
with
AD=VARPTR(X$) X$ being for instance "ABC"

Where can I find a clear complete explanation on that?

(Added to the previous one)

When I try to use the first program with VARSEG and VARPTR this time in QB4.5 ,I get several letters instead of the one I want. What can be done to elmiminate thes parasits?From where do they come?