It is currently Fri Oct 20, 2017 9:16 am

All times are UTC - 7 hours





Post new topic Reply to topic  [ 49 posts ]  Go to page Previous  1, 2, 3, 4  Next
Author Message
PostPosted: Thu May 14, 2015 12:27 pm 
Online

Joined: Sun Sep 19, 2004 11:12 pm
Posts: 19107
Location: NE Indiana, USA (NTSC)
Z80 needs twice as many cycles as 6502, but it was also usually clocked twice as fast. The VTech CreatiVision had a 2 MHz 6502. The ColecoVision and Sega SG-1000 had a 3.6 MHz Z80 but were otherwise identical in specs to the CreatiVision, down to the same amount of RAM and the same video chip. The successor to the SG-1000, the Sega Master System, had the same 3.6 MHz Z80, which was comparable to the 1.8 MHz 6502 in the NES.

If you want to get technical, you can see why clock speed doesn't matter. The NES and Super NES both run at 21.5 MHz. They just have a divider in front of the CPU core. The original Super Game Boy also runs at 21.5 MHz, with a different divider on its CPU core to produce 4.3 MHz for the Z80-like Game Boy CPU.


Top
 Profile  
 
PostPosted: Thu May 14, 2015 2:14 pm 
Offline
User avatar

Joined: Mon Sep 15, 2014 4:35 pm
Posts: 3074
Location: Nacogdoches, Texas
tepples wrote:
and the same video chip.

I thought they were generally custom made?

tepples wrote:
The NES and Super NES both run at 21.5 MHz. They just have a divider in front of the CPU core.

Why?

You know, I've noticed that both the Z80 and the 68000 are both twice as slow for the given the same frequency as their 65xx counterparts. Were the 65xx series of processors abnormally good in this regard, or where the Z80 and the 68000 just bad? Another thing: I heard that in order to change clock speed, you need a higher frequency crystal or something. Now I don't know what that means and it kind of reminds me of the crystal in a light saber, but does it cost the same amount to make a 6502 faster as a Z80 in terms of clock speed?


Top
 Profile  
 
PostPosted: Thu May 14, 2015 2:26 pm 
Offline
User avatar

Joined: Sun Jan 22, 2012 12:03 pm
Posts: 5724
Location: Canada
Espozo wrote:
tepples wrote:
The NES and Super NES both run at 21.5 MHz. They just have a divider in front of the CPU core.

Why?

Heat is usually a major limiting factor in choosing a CPU clock speed. I would imagine that they eventually managed to revise the chip to dissipate less heat at higher frequencies.

Another limiting factor is the delays inherent in how the circuitry works, though I think this has traditionally been a smaller factor than heat. It takes time for a transistor to switch from off to on. This can also be improved with progressively refined engineering.


Last edited by rainwarrior on Thu May 14, 2015 2:30 pm, edited 1 time in total.

Top
 Profile  
 
PostPosted: Thu May 14, 2015 2:29 pm 
Offline
User avatar

Joined: Sat Jan 03, 2015 5:58 pm
Posts: 367
Location: ...
I've never found the Master System to have a slow CPU, especially because of how big an interrupt routine you can fit into one frame. Speed isn't something I think you'd want to look for in 8-bit machines, anyway.


Top
 Profile  
 
PostPosted: Thu May 14, 2015 2:32 pm 
Offline
User avatar

Joined: Mon Sep 15, 2014 4:35 pm
Posts: 3074
Location: Nacogdoches, Texas
rainwarrior wrote:
Heat is usually a major limiting factor in choosing a CPU clock speed. I would imagine that they eventually managed to revise the chip to dissipate less heat at higher frequencies.

:shock: The thing doesn't generate one degree of heat! I always thought it was a cost problem. Why don't Sega arcade boards with multiple 12MHz 68000s have to worry about heat? If it's just heat, couldn't the Sega Genesis have ran faster, seeing that there were already a ton of other things with 12MHz 68000s that came out at the same time? The SNES's CPU doesn't even have a heat sink, and they're worrying about heat? I would have loved to see stronger PPUs.

Quote:
I've never found the Master System to have a slow CPU, especially because of how big an interrupt routine you can fit into one frame. Speed isn't something I think you'd want to look for in 8-bit machines, anyway.

I had always assumed the Z80 to be more than twice as slow as the 6502. I guess not.


Top
 Profile  
 
PostPosted: Thu May 14, 2015 2:51 pm 
Offline
User avatar

Joined: Sat Jan 03, 2015 5:58 pm
Posts: 367
Location: ...
Espozo wrote:
I had always assumed the Z80 to be more than twice as slow as the 6502. I guess not.

As I said, that could be true, but it doesn't cause any problems or noticeable lack of speed, so.


Top
 Profile  
 
PostPosted: Thu May 14, 2015 3:26 pm 
Online

Joined: Sun Sep 19, 2004 11:12 pm
Posts: 19107
Location: NE Indiana, USA (NTSC)
Espozo wrote:
tepples wrote:
and the same video chip.

I thought they were generally custom made?

Texas Instruments made a TI-99/4a computer. Later it made the computer's video display controller (TMS9918) available separately, and several other computers and video game consoles adopted it, including MSX, CreatiVision, ColecoVision, and SG-1000. The Master System uses a custom extended TMS9918, and we've discussed before how the NES PPU's architecture was inspired by the TMS9918's ability to run pixel-perfect Donkey Kong.

Quote:
tepples wrote:
The NES and Super NES both run at 21.5 MHz. They just have a divider in front of the CPU core.

Why?

Because transistors don't switch instantly, especially when there are lots of them that switch based on the output of other transistors. Transistors that switch faster cost more; compare the price of modern Intel CPUs rated for different speeds. A divide by 12 circuit allows a core rated for 2 MHz operation to work with the 945/44 = 21.5 MHz crystal that the PPU's color generator needs.

Quote:
You know, I've noticed that both the Z80 and the 68000 are both twice as slow for the given the same frequency as their 65xx counterparts. Were the 65xx series of processors abnormally good in this regard, or where the Z80 and the 68000 just bad?

Not "good" or "bad", just a different microarchitecture.

Quote:
Another thing: I heard that in order to change clock speed, you need a higher frequency crystal or something. Now I don't know what that means

See Crystal oscillator on Wikipedia.


Top
 Profile  
 
PostPosted: Thu May 14, 2015 4:15 pm 
Offline
User avatar

Joined: Sun Jan 22, 2012 12:03 pm
Posts: 5724
Location: Canada
Espozo wrote:
rainwarrior wrote:
Heat is usually a major limiting factor in choosing a CPU clock speed. I would imagine that they eventually managed to revise the chip to dissipate less heat at higher frequencies.

:shock: The thing doesn't generate one degree of heat! The SNES's CPU doesn't even have a heat sink, and they're worrying about heat? I would have loved to see stronger PPUs.


That is patently false. All CPUs generate heat.

The use of a heat sink is an engineering problem. Whether it's appropriate depends on how much difference it makes, how do you anchor it to the CPU, what's the added cost, etc. There's lots of situations where no practical heat sink is going to dissipate heat fast enough to make a worthwhile difference. You can't just add heat sinks until it's cool enough; it's not that simple.


People have overclocked all of these systems at some point. Look this up if you want to see how much faster they could adjust the clock speed before it reaches failure. It could be tempterature, it could be switching speed, or it could be other factors, but what you'll find invariably is that it can't really go that much faster, and is usually very limited by an overheating problem. They didn't just arbitrarily restrict the speed on you, they made it as fast as they practically could, always leaving an overhead to try to ensure it would be as reliable on a hot summer day as a cold winter night.

The exception to this rule is if a new revision of the CPU rated for higher speeds eventually becomes cheaper, later revisions of the unit might use these but be stuck at the original specified speed. (See the progression of the Nintendo DS for the opposite example of letting the specification be revised to keep up with technology.)

You did get to see stronger PPUs, you just had to wait for them to become practical. PS1 had a stronger one than SNES. XBox 360 had a stronger one than PS1. There's been progress in the technology. Why do you think we didn't have a PS4 in 1985? Do you really think it's just because somebody arbitrarily decided to run a CPU at 1/100th of the speed it was capable?


Top
 Profile  
 
PostPosted: Thu May 14, 2015 4:27 pm 
Offline

Joined: Sun Apr 13, 2008 11:12 am
Posts: 6286
Location: Seattle
Espozo wrote:
The thing doesn't generate one degree of heat! I always thought it was a cost problem.
As further evidence, game-tech.us took a bunch of thermal camera pictures of the toploader many years ago:
http://www.game-tech.us/pmwiki/pmwiki.p ... ES-101Heat
TL;DR: The CPU and PPU will easily get to a 10-40F rise over ambient temperature.


Top
 Profile  
 
PostPosted: Thu May 14, 2015 4:32 pm 
Offline
User avatar

Joined: Sun Jan 22, 2012 12:03 pm
Posts: 5724
Location: Canada
That's really neat, lidnariq.


Top
 Profile  
 
PostPosted: Thu May 14, 2015 4:59 pm 
Offline
User avatar

Joined: Mon Sep 15, 2014 4:35 pm
Posts: 3074
Location: Nacogdoches, Texas
rainwarrior wrote:
That is patently false. All CPUs generate heat.

Well, I was talking about the console itself. One reason I guess it doesn't seem nearly as hot as things nowadays if you put your hand near the vents on it is that there's no fan to blow out the hot air. Of course, it doesn't get hot enough to need a fan. (I'm sure a fan would be pretty pricey too.)

rainwarrior wrote:
Do you really think it's just because somebody arbitrarily decided to run a CPU at 1/100th of the speed it was capable?

From what I heard a little ago, that's what I thought. :oops: It didn't really make sense to me initially.


Top
 Profile  
 
PostPosted: Thu May 14, 2015 5:59 pm 
Offline
User avatar

Joined: Sat Jan 03, 2015 5:58 pm
Posts: 367
Location: ...
After some efforts I finally got a little skeleton file of my own, obviously based off of tokumaru's. I would use his but eh, for originality's sake. Or at least the sake of "disguised plagiarism"...eh, who cares.

Gotta say though, part of what was keeping me from the NES was the fact that I never could figure out what a CHR-ROM is. Now I know, but still, thought it was this random huge file you had to generate somehow.

Anyway, woot.

Code:
; nicklausw's skeleton file
; for the nes (via asm6)



; the header!
.db "NES", $1a ; ines header
.db 1 ; PRG-ROM pages (16kb)
.db $01 ; CHR-ROM pages (8kb)
.db $10 ; (no concern)
.dsb 9, $00 ; leave the trailing
; header bytes blank.




; variables.
.enum $0000
    test_var .dsb 1
.ende




; the main code
.org $c000
    ; beginning code
    Reset: jmp Reset


    ; NMI interrupts
    NMI: rti


    ; IRQ (vblank) interrupts
    IRQ: rti



; Don't touch this part
.org $fffa
.dw NMI, Reset, IRQ

; chr-rom
; .incbin "something.chr"


Top
 Profile  
 
PostPosted: Thu May 14, 2015 6:29 pm 
Offline
User avatar

Joined: Mon Sep 15, 2014 4:35 pm
Posts: 3074
Location: Nacogdoches, Texas
nicklausw wrote:
Gotta say though, part of what was keeping me from the NES was the fact that I never could figure out what a CHR-ROM is. Now I know, but still, thought it was this random huge file you had to generate somehow.

(Well, it's not on the SNES... CHR rom is tremendously more useful though. It's actually what the Neo Geo uses.)


Top
 Profile  
 
PostPosted: Thu May 14, 2015 6:37 pm 
Offline
User avatar

Joined: Sat Jan 03, 2015 5:58 pm
Posts: 367
Location: ...
Espozo wrote:
nicklausw wrote:
Gotta say though, part of what was keeping me from the NES was the fact that I never could figure out what a CHR-ROM is. Now I know, but still, thought it was this random huge file you had to generate somehow.

(Well, it's not on the SNES... CHR rom is tremendously more useful though. It's actually what the Neo Geo uses.)

Can you compress it, though? Uncompressed tiles on the Master System are a nightmare...dunno what the NES people have to deal with if they can't compress.


Top
 Profile  
 
PostPosted: Thu May 14, 2015 7:01 pm 
Offline
User avatar

Joined: Mon Sep 15, 2014 4:35 pm
Posts: 3074
Location: Nacogdoches, Texas
nicklausw wrote:
Can you compress it, though? Uncompressed tiles on the Master System are a nightmare...dunno what the NES people have to deal with if they can't compress.

Throw more memory at it. If you're not planning on making this into a real life cartridge, the sky is the limit. And of course on the Neo Geo, they weren't concerned about memory in the slightest...

Image

You can't always decompress graphics too, unless you want slowdown. One thing I wonder though: has there ever been a cartridge system that used vram that could be expanded in the cartridge? I know the NES has an ounce of VRAM, but it only really holds the pattern table? I just think CHR is the better option in terms of, basically anything but cost, (that's basically why every 80's-90's arcade board uses it, including the Neo Geo) and vram is only good for cost effective reasons. Again, vram wouldn't be a problem if you could expand it in any of these systems. I've talked about before how it seems that the SNES was originally supposed to have 128KB of vram, but I don't know if that's true or not.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 49 posts ]  Go to page Previous  1, 2, 3, 4  Next

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group