MMBasic for Windows - betas

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thwill

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Joined: 16/09/2019
Location: United Kingdom
Posts: 4048
Posted: 12:10pm 04 Mar 2022      

  al18 said  It seems Data Restore would be useful if you have a game were redefined characters are used to create a playfield. Since most Fonts have only 96 characters at a time, and you typically only wanted to define a few characters per level, the Data Restore would be useful.


Hi @al18,

MMBasic has a RESTORE command which is more powerful than that in most street BASICs and can do what I believe you are talking about.

The problem being wrangled with is that like most (all?) BASICs there is a single global "pointer" to DATA that the READ command reads from and there is no mechanism for the programmer to read that pointer and then having read it use that value to return the pointer to that state. Specifically the use of a single global pointer has limitations when for example you are partially through reading some DATA and then want to call a subroutine that needs to read some different DATA (or the same DATA from a different point).

This is crazy code, but does illustrate the point that whenever exiting mad() we want the data pointer to be the same as it was when we entered mad():

Option Base 0
Option Default None
Option Explicit On

Restore my_data
mad(5)
End

Sub mad(depth%)
   If depth% = 0 Then Exit Sub
   Local data_pos% = Peek(DataPos) ' Cache the data pointer.
   Local s$
   Restore my_data
   my_label: ' There seems to be an issue with using recursion and DO/FOR loops ?
             ' Might be MMB4L specific.
       Read s$
       If s$ = "" Then
           Print
           Poke DataPos data_pos% ' Restore the data pointer.
           Exit Sub
       EndIf
       Print Space$(5 - depth%) Str$(depth%) s$ " ";
       If depth% > 1 Then Print
       mad(depth% - 1)
   Goto my_label
End Sub

my_data:
Data "A", "B", "C", "D", "E", ""


Note that PEEK(DATAPOS) and POKE DATAPOS don't currently exist, they are my proposed solution to the problem.

Best wishes,

Tom
Edited 2022-03-04 22:17 by thwill