execbase=4

NOB · 13 February 2006, 18:47

The ExecBase value is 4.
Why did the commodore engineers took the 4?
Did anyone know that?

Ray Norrish · 13 February 2006, 20:44

I presume 0 was used / reserved for something, and then they didn't bother.

Galahad/FLT · 14 February 2006, 00:03

0 was used for Kickstart crash marker. When you get a Guru Mediation crash, the word 'HELP' is written at address 0, and seeing as that ASCII text would be equivalent of a longword, the next free space is address 4.

NOB · 14 February 2006, 18:41

@Ray
@Galahad

Thank you for the infos.

bburtonpa · 27 April 2006, 21:58

The reason (Abs)ExecBase = 4 is because 0 is NULL in 'C', maybe ASM too. If I remember correctly, all library entry points had to be on four byte boundries (memory location evenly divisible by four), that's also why it's not 1 or 2 or 3...

Ray Norrish · 27 April 2006, 23:31

only cos 4 is longword (which is minimum for proper addressing). Galahad's explanation is correct.

13 February 2006, 18:47	#1
NOB Zone Friend Join Date: Aug 2005 Location: Germany Age: 52 Posts: 424	execbase=4 The ExecBase value is 4. Why did the commodore engineers took the 4? Did anyone know that?

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13 February 2006, 20:44	#2
Ray Norrish Registered User Join Date: May 2005 Location: Cheshire, UK Age: 56 Posts: 322	I presume 0 was used / reserved for something, and then they didn't bother.

14 February 2006, 00:03	#3
Galahad/FLT Going nowhere Join Date: Oct 2001 Location: United Kingdom Age: 50 Posts: 8,994	0 was used for Kickstart crash marker. When you get a Guru Mediation crash, the word 'HELP' is written at address 0, and seeing as that ASCII text would be equivalent of a longword, the next free space is address 4.

14 February 2006, 18:41	#4
NOB Zone Friend Join Date: Aug 2005 Location: Germany Age: 52 Posts: 424	@Ray @Galahad Thank you for the infos.

27 April 2006, 21:58	#5
bburtonpa Amiga user since 1985 Join Date: Apr 2005 Location: Middlesex, NJ USA Posts: 14	The reason (Abs)ExecBase = 4 is because 0 is NULL in 'C', maybe ASM too. If I remember correctly, all library entry points had to be on four byte boundries (memory location evenly divisible by four), that's also why it's not 1 or 2 or 3...

27 April 2006, 23:31	#6
Ray Norrish Registered User Join Date: May 2005 Location: Cheshire, UK Age: 56 Posts: 322	only cos 4 is longword (which is minimum for proper addressing). Galahad's explanation is correct.

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