roondar |
07 July 2019 14:58 |
Quote:
Originally Posted by Foebane
(Post 1331107)
Actually, I thought the guy was trashing the A1200 and talking crap about the A500 as well, because he said "OCS". It wasn't easy to tell, with his broken English and everything.
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I'm pretty sure the gist of that part of his post was that the A500/OCS was 'unfinished' because A500 trap door RAM was as slow as Chip RAM (due to them sharing the same bus) and that the chip set registers were on the same bus as Chip RAM. Which means they were also slowed down.
Of course, in reality the A500 was essentially a heavily cost cut A1000 with minor changes. This was needed as the A1000 was reportedly really expensive to make and as such couldn't be offered at an appealing price point for mass market sales (which is what Commodore wanted/needed - they wanted the Amiga to be the next C64 and were losing money because of the A1000's low sales at the time). The trap door RAM slot was one of those changes and as I understand it, the reason it works the way it does is because it was much, much cheaper to do it this way compared to what Sandruzzo suggested.
Plus, if you really wanted fast memory the side slot of the A500 already offered this option. It's just that everyone who expanded their A500 elected to use the cheaper trap door memory.
Later revision A500's actually can access trapdoor memory through the chipset (making it essentially chip RAM), though the processor doesn't understand this (due to how the memory is mapped) and as such still reports it as fast RAM. This is why the Indivision ACA500/ACA500+ could give you 1MB chip RAM without altering anything on the main board. But this is merely cosmetic - you can write programs accessing 1MB of chip memory on any A500 with the correct Agnus and board revision (i.e. all with revision 5+ I think).
Though obviously you'd lose compatibility with the older A500's if you do so, which is probably why no one did this.
As for the registers, I'm not a hardware engineer so I may have this wrong. But as I understand it, what Sandruzzo asks for here would've essentially required two busses on the A500 board: one for the registers, one for chip RAM. Well that or have a 'double ported bus' such as used by VRAM. These things might be possible, but would be rather expensive.
The real irony here being that the A1200 trap door slot fixed all of these things as well as essentially remove most of the chip RAM slow down when using higher numbers of colours in games, but he still doesn't like it ;)
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