CPU config

Eamoe · 01 April 2011, 09:04

Hi all,

questions questions questions...

I've just read a bit about CPU configuration. Yet I don't seem to understand it very well. Could someone explain what are the following:

- inst cache / data cache
- burst
- fastROM

What are the possible gains of setting these on or off? When should they or should they not be set? Does setting FastROM on mean that I'll gain some chip RAM? (I do have enough fast RAM to store the KS)

Note 1: I have a 68020 / 68882.
Note 2: I've found this: http://www.whdload.de/docs/en/cache.html.

rkauer · 04 April 2011, 03:04

Inst cache: instruction cache. Present on 020 CPU onwards.

Data cache. Present on 030 CPU onwards.

Burst: secondary access to memory on the middle of a instruction processing. Only present on 040 onwards.

FastROM: copy the ROM contents to the FAST RAM and all ROM accesses will be from RAM (faster). Only on systems with MMU and some FAST RAM (more than the size of the ROM, obviously).

Will "eat" the size of the ROM from FAST. Good speed gain (around 15~20%) while in Workbench and applications. Games don't take advantage of it because no ROM accesses.

StingRay · 04 April 2011, 14:21

Quote:

Originally Posted by rkauer

Burst: secondary access to memory on the middle of a instruction processing. Only present on 040 onwards.

Not true. 68030 has burst mode too. Some more info:

Quote:

The cache lines are used in conjunction with the 68030's and 68040's
burst mode. Normally, the 68030 fills its caches one entry at a
time. While in burst mode, the CPU fills its caches a whole cache
line at a time. This helps to reduce the number of cache misses.

On the 68030, it is possible to turn burst mode on and off
independently of its caches. If the 68030 cache is on and burst mode
is off, the 68030 can fill its cache a single long word at a time,
rather than the four words at a time it would do in burst mode. The
68040 is different. On the 68040, the only way to turn on burst mode
is to turn on the cache, so there is no way to prevent a burst access
when using the cache. The 68040 always fills a whole cache line at a
time.

Source:
The 68030 and 68040 on the Zorro III Bus by Michael Sinz

Loedown · 06 April 2011, 12:28

Quote:

Originally Posted by rkauer

Will "eat" the size of the ROM from FAST. Good speed gain (around 15~20%) while in Workbench and applications. Games don't take advantage of it because no ROM accesses.

C64 used Kernal ROM with small routines designed to make programmer's life a little easier, didn't some game writers on the Amiga use a similar technique?

Jope · 06 April 2011, 20:02

Not really, as it is not designed to be called directly.

Parts of the multitasking operating system run from ROM, but that's libraries and drivers that you call via OS calls instead of things that you can just jump to without any further thought.

Some 1.x era SW used direct ROM jumps and naturally those broke every time the kickstart was updated.

FWIW, most of the more involved C-64 stuff chucks the KERNAL out.

StingRay · 06 April 2011, 20:05

Quote:

Originally Posted by Loedown

C64 used Kernal ROM with small routines designed to make programmer's life a little easier, didn't some game writers on the Amiga use a similar technique?

No since almost all games disable the operation system and don't use any libraries etc. in ROM. Some games might access the ROM to get "random" values but other than that, the system routines aren't used in 99% of all hardware banging games.

rkauer · 07 April 2011, 06:17

I think the only one library called from games is the lowlevel.library (that's used by the joystick, which in turn can be with one, two or multiple buttons).

Even on the above case, normally the games have their own version of the lowlevel.library to work from.

Loedown · 07 April 2011, 13:27

@Jope & Stingray

I'd actually forgotten that the Kernal ROM can be dropped out of the system, RAM is used to take up the space if the Kernal ROM is dropped, the same could be said for a few other devices too in the address space.

Eamoe · 07 April 2011, 23:21

Very interesting indeed.

So, if I get it right, no fastrom with my 020...

Do you guys know of a good 030 accelerator card for the 500, that has limited footprint and some MMU?

rkauer · 08 April 2011, 07:33

VXL30 among other internal ones.

Do a read on the full 030 list: http://amiga.resource.cx/exp/search....&base=dec&pid=

Eamoe · 08 April 2011, 13:54

Wow... sure could use one Viper 530 or VXL30! I'm litterally droolin' on my keyboard!

01 April 2011, 09:04	#1
Eamoe Stunt Car Racer Vet Join Date: Mar 2006 Location: Vevey/Switzerland Age: 44 Posts: 145	CPU config Hi all, questions questions questions... I've just read a bit about CPU configuration. Yet I don't seem to understand it very well. Could someone explain what are the following: - inst cache / data cache - burst - fastROM What are the possible gains of setting these on or off? When should they or should they not be set? Does setting FastROM on mean that I'll gain some chip RAM? (I do have enough fast RAM to store the KS) Note 1: I have a 68020 / 68882. Note 2: I've found this: http://www.whdload.de/docs/en/cache.html.

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04 April 2011, 03:04	#2
rkauer I hate potatos and shirts Join Date: Oct 2007 Location: Sao Leopoldo / Brazil Age: 58 Posts: 3,482	Inst cache: instruction cache. Present on 020 CPU onwards. Data cache. Present on 030 CPU onwards. Burst: secondary access to memory on the middle of a instruction processing. Only present on 040 onwards. FastROM: copy the ROM contents to the FAST RAM and all ROM accesses will be from RAM (faster). Only on systems with MMU and some FAST RAM (more than the size of the ROM, obviously). Will "eat" the size of the ROM from FAST. Good speed gain (around 15~20%) while in Workbench and applications. Games don't take advantage of it because no ROM accesses.

06 April 2011, 20:02	#5
Jope - Join Date: Jul 2003 Location: Helsinki / Finland Age: 43 Posts: 9,915	Not really, as it is not designed to be called directly. Parts of the multitasking operating system run from ROM, but that's libraries and drivers that you call via OS calls instead of things that you can just jump to without any further thought. Some 1.x era SW used direct ROM jumps and naturally those broke every time the kickstart was updated. FWIW, most of the more involved C-64 stuff chucks the KERNAL out.

07 April 2011, 06:17	#7
rkauer I hate potatos and shirts Join Date: Oct 2007 Location: Sao Leopoldo / Brazil Age: 58 Posts: 3,482	I think the only one library called from games is the lowlevel.library (that's used by the joystick, which in turn can be with one, two or multiple buttons). Even on the above case, normally the games have their own version of the lowlevel.library to work from.

07 April 2011, 13:27	#8
Loedown Precious & fragile things Join Date: Feb 2009 Location: Victoria, Australia Posts: 1,946	@Jope & Stingray I'd actually forgotten that the Kernal ROM can be dropped out of the system, RAM is used to take up the space if the Kernal ROM is dropped, the same could be said for a few other devices too in the address space.

07 April 2011, 23:21	#9
Eamoe Stunt Car Racer Vet Join Date: Mar 2006 Location: Vevey/Switzerland Age: 44 Posts: 145	Very interesting indeed. So, if I get it right, no fastrom with my 020... Do you guys know of a good 030 accelerator card for the 500, that has limited footprint and some MMU?

08 April 2011, 07:33	#10
rkauer I hate potatos and shirts Join Date: Oct 2007 Location: Sao Leopoldo / Brazil Age: 58 Posts: 3,482	VXL30 among other internal ones. Do a read on the full 030 list: http://amiga.resource.cx/exp/search....&base=dec&pid=

08 April 2011, 13:54	#11
Eamoe Stunt Car Racer Vet Join Date: Mar 2006 Location: Vevey/Switzerland Age: 44 Posts: 145	Wow... sure could use one Viper 530 or VXL30! I'm litterally droolin' on my keyboard!

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