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Old 18 December 2019, 23:08   #1
sparhawk
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movec on 68000

I'm converting ministartup from here: http://stingray.untergrund.net/MiniStartup.s and stumbled over this part:


Code:
.GetVBR	move.l	a5,-(a7) 	moveq	#0,d0			; default at $0 	move.l	$4.w,a6 	btst	#0,296+1(a6)		; 68010+? 	beq.b	.is68k			; nope. 	lea	.getit(pc),a5 	jsr	-30(a6)			; SuperVisor() .is68k	move.l	(a7)+,a5 	rts  .getit	movec   vbr,d0 	rte				; back to user state code

It executes the "movec" privileged instruction which is AFAIK an 68010+ instruction, right? So I get an error from vasm because I'm targetting 68000:


instruction not supported on selected architecture



And I wonder how to work around this, as I'm not sure what exactly it does.
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Old 18 December 2019, 23:24   #2
Galahad/FLT
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Change the line MOVEC VBR,d0 to this:

dc.l $4E7A0801

VBR = Vector Base Register.

On a 68000 the VBR cannot be moved, so the base address is $0. The Vector Base is the area of memory where the vectors are stored, so thats BUS ERROR etc and of course the interrupts.

On 68010+ the Vector Base can be repositioned, and all those interrupts will also be moved.
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Old 18 December 2019, 23:40   #3
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Thanks. I enabled 68010 instruction set. I was trying to find the encoding for the instruction.
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Old 19 December 2019, 14:28   #4
DanScott
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this should do it:

mc68020
movec vbr,d0
mc68000
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Old 19 December 2019, 14:32   #5
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Thanks! I was looking on how to enable an architecture for only some instructions, but I haven't found this in the docs. This is much better, as I want to target 68000 and I wouldn't like to slip in other instructions by accident, as it would be possible at the moment.
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Old 19 December 2019, 14:40   #6
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Quote:
Originally Posted by sparhawk View Post
Thanks! I was looking on how to enable an architecture for only some instructions, but I haven't found this in the docs. This is much better, as I want to target 68000 and I wouldn't like to slip in other instructions by accident, as it would be possible at the moment.
That specific movec in Stingrays mini startup code wont be executed on 68000 as he does a processor check before.
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Old 19 December 2019, 15:23   #7
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Yes, But it should be there, in case it is executed on a different machine, right? So the code wouldn't work, if the program would be started on some A1200 or so.
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Old 19 December 2019, 16:08   #8
DanScott
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ministartup works perfectly well on all Amigas / processors / configs

On 68000, the movec part is never called, so it's just in effect some bytes in memory
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Old 19 December 2019, 16:44   #9
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Yes, thats why I left it in. I could have removed that part, but then it wouldn't work properly on other Amigas.
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