Subject: Re: ROM Maps
To: Dave Huang <khym@bga.com>
From: Daniel R. Killoran,Ph.D. <drk@shore.net>
List: port-mac68k
Date: 03/01/1999 08:53:39
>On Sun, 28 Feb 1999, Daniel R. Killoran,Ph.D. wrote:
>> That's impossible! Apple could have ADDED traps (and did), but if they
>> moved them it would break most existing programs!
>
>No, it's not like on the Apple II where programs would jump to the ROM
>routine directly. On the Mac, programs use the trap number (which does
>remain constant across all ROMs). The ROMs then handle figuring out
>where the actual address of the routine is.

This is a distinction without a difference - on the Mac, the hardware
fetches the address from the trap table when it hits an A-instruction.
There is no practical difference between this and a jump to a transfer
table except that the registers get saved.

Dan Killoran

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