[v1.2 only] Crash on changing graphics page after changing PCjr/Tandy video memory size

[32bitkid]

Bug report

Problem
When using pcjr/tandy graphics modes and using clear to clear video memory while on a graphical screen, pcbasic does not reset the video mode and will crash if you attempt to access the newly available screen page using screen 4,,1,1 (Rather than returning Illegal function call like it would before the clear command)

On an actual PCjr, when the clear command is not a noop, it resets the screen to mode 0. I do not have a Tandy available for testing.

Program

$ pcbasic --preset=pcjr

10 screen 4
20 clear ,,,32768!
30 screen 4,,1,1

Crash log

EXCEPTION                                                                       
:SCREEN 4,0,1,1                                                                                                                                                 
pcbasic.statements:169, parse_statement                                         
    None                                                                        
pcbasic.statements:2501, exec_screen                                            
    None                                                                        
pcbasic.backend:1040, screen                                                    
    None                                                                        
pcbasic.backend:1187, set_page                                                  
    None                                                                        
IndexError: list index out of range                                             

Notes

PC-BASIC version: 1.2.14 and 2.0.2beta
Operating system version: MacOS 10.14.5

[robhagemans]

Thank you for reporting this! Looks like there is an issue with keeping track of the video page when the number of pages changes.

Also thanks especially for confirming PCjr behaviour, as I only have access to emulators...

[robhagemans]

Reproduced on both 1.2 and 2.0 branches

[robhagemans]

Tried this on DOSBox with Tandy emulation and Tandy GW-BASIC and it hangs, so unclear what real Tandy hardware does here. I'm going with dropping to text as with the pcjr

[robhagemans]

This is now fixed on develop for next release on 2.0 branch, hence marking as 1.2 only

[robhagemans]

Fixed on release-1.2 branch by commit 4298390