[Coco] CoCo BASIC

[Arthur Flexser]

Steve, I just ran the following Basic program in the Mocha online emulator.

10 DIM A(4,5)
20 PRINT HEX$(VARPTR(A(1,1)))
30 PRINT HEX$(VARPTR(A(2,1)))
40 PRINT HEX$(VARPTR(A(1,2)))

The output was:

1E76
1E7B
1E8F

This tells me that the array elements are stored consecutively (in the
array storage area following the end of the program) as 5-byte quantities,
with the first subscript varying fastest:

A(0,0), A(1,0), A(2,0), A(3,0), A(4,0),    A(0,1), A(1,1), A(2,1), A(3,1),
A(4,1),    A(0,2), A(1,2), A(2,2), A(3,2), A(4,2),    A(0,3), A(1,3) ....,
etc.

This follows from the fact that the output shows that A(1,1) and A(2,1) are
stored 5 bytes apart, while A(1,1) and A(1,2) are 25 bytes apart.

Further peeking showed that the array area pointer at $1D-1E points to
$1E4F, which, if you add 9 bytes for the header, agrees with the location
of A(0,0), which is stored at $1E58,

Art



On Tue, Mar 23, 2021 at 8:51 PM Steve Ostrom <smostrom7 at comcast.net> wrote:

> I’m trying to understand how BASIC handles arrays.
>
> The routine to evaluate an array variable starts at location $B401.  The
> start address of the table for declared arrays is stored at $001D-$001E.
> The end of this table +1 is stored at $001F-$0020.  The array flag is
> stored at $0005.
>
> Each element in an array is stored as a 5-byte value.  In the array table,
> each array is preceded by a header block of 7 bytes for single dimensioned
> arrays or 9 bytes for a two dimensional array.  Those bytes are made up of
> 2 bytes for the array name, 2 bytes containing the length of the array plus
> the header, the fifth byte holds the number of dimensions, followed by 2
> bytes for each dimension that contains the length of the dimension.
>
> My guess is that the values for a single-dimensioned array are stored
> sequentially in the memory location reserved for arrays.  But how are the
> values for multidimensional arrays stored?  Sequentially by the first
> dimension, followed by values for the second dimension?
>
> My goal is to set up arrays in BASIC, then access those arrays via
> assembly language.
>
> Thanks again for the help.  Relearning this stuff has been fun, and good
> exercise for my 72 year old brain !!   😊
>
> --- Steve ---
>
>
>
> Sent from Mail for Windows 10
>
>
>
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