Print Page - Why WAIT in basic freezed?

Title: Why WAIT in basic freezed?
Post by: McArti0 on 01:08, 04 February 22

Why:

WAIT &f500,1 work.

But WAIT &f500,1,0 ... don't work? (freezed)

WAIT &f500,1,255 don't work too.

Manual say:

W A I T <port number, , <mask,[, <inversion,]

" The value read is eXclusive 0Red with the <inversion, and then ANDed with the <mask,"

Title: Re: Why WAIT in basic freezed?
Post by: gurneyh on 07:40, 04 February 22

hi,

you do not necessarily need to use inversion

Code Select

WAIT &F500,1 is ok.

https://www.grimware.org/doku.php/documentations/software/locomotive.basic/startdo=export_xhtml#wait (https://www.grimware.org/doku.php/documentations/software/locomotive.basic/startdo=export_xhtml#wait)

Title: Re: Why WAIT in basic freezed?
Post by: pelrun on 08:26, 04 February 22

His point is that even using an inversion of 0 which should be completely identical to omitting the parameter, WAIT behaves differently. Sounds like a bug in BASIC to me.

Title: Re: Why WAIT in basic freezed?
Post by: pelrun on 08:56, 04 February 22

Confirmed - in both BASIC 1.0 and 1.1 if you put anything in the inversion parameter then the mask gets incorrectly set to 0, making it never return :picard2:

More details: Using the disassembly of BASIC 1.1 at https://github.com/Bread80/Amstrad-CPC-BASIC-Source/blob/ae70de4838bbfda0b53d45ff05ff61bd44f0861d/PeekPokeIOBarCall.asm#L40 (https://github.com/Bread80/Amstrad-CPC-BASIC-Source/blob/ae70de4838bbfda0b53d45ff05ff61bd44f0861d/PeekPokeIOBarCall.asm#L40)

The problem is that the call to eval_expr_as_byte_or_error in line 60, which is used to read in both the mask and inversion values, corrupts DE. Since the mask value is stored in D, it gets overwritten by the high byte of the inversion parameter (which is guaranteed to be 0 or it would have thrown an Improper Argument error and not returned.)

Title: Re: Why WAIT in basic freezed?
Post by: eto on 09:38, 04 February 22

Quote from: McArti0 on 01:08, 04 February 22WAIT &f500,1,255 don't work too.

does WAIT &f500,0,1 work?

Title: Re: Why WAIT in basic freezed?
Post by: McArti0 on 10:12, 04 February 22

Quote from: eto on 09:38, 04 February 22
does WAIT &f500,0,1 work?

No :-X

Adding XOR destroys AND

Title: Re: Why WAIT in basic freezed?
Post by: pelrun on 10:26, 04 February 22

Quotedoes WAIT &f500,0,1 work?

Given the way WAIT works, putting 0 in the mask field will ALWAYS lock the machine up, even when the inversion field is missing. You're asking the machine to wait until a bit is set, but clearing all the bits before you test. So technically, it "works" because locking up is exactly what you asked it to do.

Title: Re: Why WAIT in basic freezed? [solved- BASIC BUG]
Post by: McArti0 on 10:41, 04 February 22

Quote from: gurneyh on 07:40, 04 February 22
hi,

you do not necessarily need to use inversion

but I want to find end time of H-Sync

Lord Alan... , give my money back ;) :D

Title: Re: Why WAIT in basic freezed?
Post by: eto on 14:12, 04 February 22

Quote from: pelrun on 10:26, 04 February 22Given the way WAIT works, putting 0 in the mask field will ALWAYS lock the machine up

:doh: argl... sure... AND 0

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General Category => Programming => Topic started by: McArti0 on 01:08, 04 February 22