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Optimizing for FastRAM & 68060 CPU

DrWatson's profile picture
Published in 
atari
 · 18 May 2024

Well, so it happend, ct60 arrived to our hands =) Short time after receiving my CT60 I've got some nice c2p source from Evil/DHS by Michael Kalms aka Scout/Appendix where I learned some things about 060 so why not write it here:)

Burst mode

I was very surprised when I didn't find 'burst mode' bit in CACR register. But this doesn't mean 060 has no burst... 060 operates in burst mode all the time :) And what is this burst mode? If you read my article about 030 timing you know how it works in the 030 cache: every word or long read has its place in data cache (unless the data cache isn't disabled and/or frozen of course)... so if you want to load let's say 32 bytes into data cache you have to do on 030:

        tst.w   0(a0)           ;  4 bytes loaded 
tst.w 4(a0) ;+ 4 bytes
tst.w 8(a0) ;+ 4 bytes
tst.w 12(a0) ;+ 4 bytes
:
:
tst.w 28(a0) ;+ 4 bytes = 32 bytes

Since every data entry is stored as a long... you can use the advantage of misaligned operands:

        tst.w   3(a0)           ;  4+4 bytes loaded 
tst.w 11(a0) ;+ 4+4 bytes
tst.w 19(a0) ;+ 4+4 bytes
tst.w 27(a0) ;+ 4+4 bytes = 32 bytes

Since we're filling two entries at once (because of misaligned words). And now the Burst Mode: this one allows you to load 16 bytes at once if your cache line is 'clean', that means all entries are marked as 'invalid' and your data is read from a 16 bytes boundary:

        tst.w   (a0)            ;  16 bytes loaded 
tst.w 16(a0) ;+ 16 bytes = 32 bytes

And we can again use advantage of misaligned operands:

        tst.w   0*16+15(a0)     ; 32 bytes loaded 
tst.w 2*16+15(a0) ; next 32 bytes loaded

Cool isn't it? :) By the way, you can use this trick on CT2, too, since CT2 has FastRAM & burst support for it. But don't forget to enable the previously mentioned bit in the CACR !!!

Writing to ST RAM

Yeah, yeah... we have got superb 68060 CPU, superb FastRAM and still we have to do what? Write to ST RAM! Now someone could ask if 68060 and FastRAM helps in this area, too. So, for very curious people: YES, IT DOES :) How?

1. Store Buffer

Even if our 8 KB data cache is a lot of space, for copying thousands of bytes It isn't very useful :) And so here comes our store buffer into play: it's a four entry (that means 4 longs) first-in-first-out buffer used by writing to slow memory. So, if we want to write a word or long to memory and the databus is still used by the previous memory write, this value will be stored in this buffer and the program will continue to the next (hopefully not memory operate) instruction.

2. Instruction overlapping

I touched this topic in 030 timing article a little bit: If your code isn't only about writing to ST RAM, you can use this very nice trick with fantastic results. I mean here a famous chunky to planar routines of course. Let's make some analysis:

For 320*240/TC you need to transfer/clear 320*240*2 = 153600 bytes what is 76800 words. If one word takes 4 cycles to write to memory, we need 307200 cycles. And most demos didn't use 'true' truecolor: they used lookup table for 256 colours + additional values for lighting, shading or pixel overlapping...

What about 256 colour modes? On standard Falcon we can't use them because of ... bitplanes. Simply, without FastRAM you have to:

  • clear chunky buffer in ST RAM (320*240 bytes = 38400 words)
  • do some nice 3D stuff (variable amount of writes)
  • copy from chunky buffer to screen memory (2*38400 words since both chunky buffer and screen are in ST RAM)

This gives us 3*38400 words what is 3*38400*4 = 460800 cycles and still without instruction timings.. so it's slower than TC..

OK, but FastRAM comes into play! The situation looks much much better:

  • clearing chunky buffer in FastRAM (19200 longs)
  • do some nice 3D stuff (still in FastRAM)
  • c2p conversion (19200 longs to transfer = 38400 words)

So... if one write to SDRAM is one 66.666 MHz clock cycle what is 16/66.666 = 0.24 of one 16 MHz clock cycle we get:

19200*0.24 + 38400*4 = 158208 cycles!

Let's compare:

  • 320*240/TC: 307 200 cycles + reads from lookup table
  • 320*240/256: 158 208 cycles

Maybe you ask why I'm so sure c2p conversion will not take some cycles ;) It's because of instruction overlapping. If you write a long to ST RAM (typically c2p where we are writing longs) it takes eight 16 MHz cycles:

2*4*(1/16000000) / (1/66666000) ~ 33 66.666 MHz cycles between each c2p pass. And be sure in this time you can do everything :)

Here we see that idea of putting our truecolor screen into FastRAM has practically no sense - ok, we have our buffer in SDRAM:

  • clearing of buffer: 320*240*2 = 38400 longs
  • doing stuff in FastRAM...
  • copying to ST RAM: 38400 longs to transfer = 76800 words

38400*0.24 + 76800*4 = 316416... we didn't help ourselves very much... 2 times slower than 256 colours mode....

Caches

Only some words on this topic: unroll your loops !!!!!!!!!!!!!! =) And for ST RAM operations... try to optimize a program pipeline to the max...

Superscalar architecture

People, this thing rules =) It's a little bit similar to the DSP pipeline, but with much more freedom. Here's a copy&paste from one mail by Amiga guy Thomas Richter:

Actually, the '060 UM is sufficient in this topic. The '060 has two ALUs of which one has only a restricted instruction set (the sOEP). Most *simple* operations can run in parallel in the pEOP and sOEP provided the results don't depend on each other (and provided you don't trip on a bug in the '060 of which - unfortunately - there are some).

Thus,

add.l d0,d1
add.l d2,d3

can be executed in parallel since "add" can be executed in both ALUs, and the source of the second instruction does not depend on a result of the first.

Instructions as "sOEP|pOEP" run on both ALUs. Those marked as "pOEP" can only run on the primary ALU and hence may cause stalls. Further-more, the FPU runs in parallel with the integer unit, it makes quite some sense to 'fire off' the FPU and to perform integer arithmetic while the FPU is busy.

Thus, programming hint: Try to 'interleave' instructions from two separate instruction pipelines to keep both ALUs busy. For example, if you have tight inner loops, unroll the loop (if possible) into two parallel instruction streams.

I proved this to myself by modifying that c2p routine which Evil sent me and I have to say, it's faster! Even with incredible slow ST RAM writes!

And that is it... just short overview if you are too lazy to read Motorola's docs ;) What to say at the end... make sure you have enabled instruction & data & branch cache, enabled FIFO buffer for data cache and enabled "superscalar mode" in PCR !

I attached to this article mentioned c2p routine, I don't know a faster one at this time ;)

CT60 rules !!!!!!!!!!!! =)

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