The 8 Bit tech comparison Thread

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NorthWay
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Post by NorthWay » Mon Aug 13, 2007 5:03 pm

ivarf wrote:BTW, how many bits do the ZX Spectrum use per pixel?
Not enough?

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Post by NorthWay » Mon Aug 13, 2007 5:13 pm

I'd like a good technical description on what makes the BBC and CPC display have different screen colours, size, and modes with the same 6845 chip.

Also, are there any disassemblers that support the BBC executable file format? (What is it anyway, anyone got an URL to describe it?)

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Post by Antiriad2097 » Mon Aug 13, 2007 5:56 pm

ivarf wrote:BTW, how many bits do the ZX Spectrum use per pixel?
One and a bit. Each block of 8 pixels is an 8 bit byte, then there's the colour overlay info which is tiled to an 8x8 grid. Can't recall the exact spec, but I think its one 8 bit byte per 8x8 character for the colour data.
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Post by Dudley » Tue Aug 14, 2007 5:27 am

Antiriad2097 wrote:
ivarf wrote:BTW, how many bits do the ZX Spectrum use per pixel?
One and a bit. Each block of 8 pixels is an 8 bit byte, then there's the colour overlay info which is tiled to an 8x8 grid. Can't recall the exact spec, but I think its one 8 bit byte per 8x8 character for the colour data.
Which is, I believe, where the colour clash comes in.
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Post by matt » Tue Aug 14, 2007 6:03 am

We shouldn't forget the Amstrad Plus machines too.

Image

Image

Enhanced versions of the CPC machines with 4096 colours, DMA sound, hardware sprites, smooth scrolling, etc.
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TMR
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Post by TMR » Tue Aug 14, 2007 7:18 am

Antiriad2097 wrote:
ivarf wrote:BTW, how many bits do the ZX Spectrum use per pixel?
One and a bit. Each block of 8 pixels is an 8 bit byte, then there's the colour overlay info which is tiled to an 8x8 grid. Can't recall the exact spec, but I think its one 8 bit byte per 8x8 character for the colour data.
Yup, three bits of a byte specify which of eight colours to use for background, another three give the foreground, one bit says if "bright" is turned on for that attribute cell (both colours will be affected, you can't use dark blue for one and light blue for the other) and one for flash.
Dudley wrote:Which is, I believe, where the colour clash comes in.
Colour clash more about trying to move an object that's using one set of colours into the colour cells where another object is already writing something else to the attribute map; the new arrival's colours (or if they're moving, the last object drawn) overlap into the other object. Games that have flickering colour clash where it changes back and forth between two colours aren't synchronised to the display refresh, sometimes the scan passes over them when they're one colour, other times they're the other.

The Spectrum isn't alone in having clash, the VIC 20, C16/Plus/4 have the same issues because they have to use software sprites as well and it's possible to get clash out of the C64 or Atari 8-bit using software sprites too (the latter is a bit harder but can be done =-)

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Post by ivarf » Tue Aug 14, 2007 7:28 am

I tried some old Amsoft games on my CPC a few days ago, and several of the "sprites" (hohum...) in those games colourclashed with other sprites or background

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Post by TMR » Tue Aug 14, 2007 7:42 am

batman877 wrote:I think threads like this really take the fun out of Retro gaming.

Only my opinion of course ...
Sorry, missed this post (spent all of yesterday sidetracked with some PHP, been arguing with the OSG back engine again) but i do have something to say; this thread has evolved from other people wanting to discuss the technical nature of various computers. Whilst you may not see why, it certainly does not take the fun out of retro gaming for those people in the same way that knowing how magic tricks are done doesn't take the fun out of watching a performance.

It's a bit like the difference between merely watching a film on DVD and enjoying the extras, i'm very much in the latter camp because i like to know how things i'm seeing are done. If you don't enjoy that kind of behind the scenes view of things, might be better to avoid this thread in the same way i'm avoiding the comparison thread after all the complaints about tech discussion - things could possibly get a lot more technical. i don't see any posted disassemblies yet. =-)

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Post by TMR » Tue Aug 14, 2007 7:50 am

ivarf wrote:I tried some old Amsoft games on my CPC a few days ago, and several of the "sprites" (hohum...) in those games colourclashed with other sprites or background
Actually, that's not the same thing; it's down to how the CPC handles colours and the way the software sprites were written in; in the lowest res mode, the CPC uses four bits per pixel whilst in the four colour mode it's using two bits (in both cases, any colour from the palette of 27 can be assigned to any of those bit combinations). Software sprites can be written to display in a number of ways, the "best" is the most complex because it involves reading the background where the sprite will be going, ANDing a mask in, ORAing the sprite data and writing back to the screen.

If the machine is running low on CPU time or there's no space to stash the background data for removing the sprites later, that can be reduced to read / exclusive OR / write (and suddenly the same routine does double duty since it can remove the sprites as well as write them) but should these sprites pass over background detail or each other they start generating... interesting numbers and that's what you're seeing.

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Post by Antiriad2097 » Tue Aug 14, 2007 8:56 am

TMR wrote:
Antiriad2097 wrote:
ivarf wrote:BTW, how many bits do the ZX Spectrum use per pixel?
One and a bit. Each block of 8 pixels is an 8 bit byte, then there's the colour overlay info which is tiled to an 8x8 grid. Can't recall the exact spec, but I think its one 8 bit byte per 8x8 character for the colour data.
Yup, three bits of a byte specify which of eight colours to use for background, another three give the foreground, one bit says if "bright" is turned on for that attribute cell (both colours will be affected, you can't use dark blue for one and light blue for the other) and one for flash.
I'm quite pleased with myself for getting that right - it was a bit of a guess based on vague memories and theory :D
Thanks for validating that for me TMR.
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Post by TMR » Tue Aug 14, 2007 10:56 pm

Antiriad2097 wrote:I'm quite pleased with myself for getting that right - it was a bit of a guess based on vague memories and theory :D
Thanks for validating that for me TMR.
Y'welcome. =-)

The C64 uses a similar system in hires mode, except it uses four bits per colour so it gets sixteen possible colours for each on or off pixel in each attribute cell - the Plus/4 takes it a few bits further, it uses two bytes instead of one per cell and seven bits of those bytes to represent the colour values so two colours per cell again but they can be anything from the full 121 colour palette.

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Re: The 8 Bit tech comparison Thread

Post by GarryG » Wed Aug 15, 2007 8:56 am

I thought most surveys and stuff on this type of thing agreed that the Enterprise was generally the best 8Bit machine from a technical viewpoint?

Although I generally fall asleep before reaching the end of things like that…


Technical stuff, if you are remotely interested:
NAME Enterprise 64 / 128
MANUFACTURER EnterpriseTYPE Home Computer
ORIGIN United Kingdom
YEAR 1985
BUILT IN LANGUAGE IS-Basic (ROM cartridge)
KEYBOARD Complete keyboard with function keys and built-in joystick
CPU Zilog Z80A
SPEED 4 MHz
CO-PROCESSOR NICK (video), DAVE (sound)
RAM Enterprise 64 : 64 KB (50 KB available)
Enterprise 128 : 128 KB
VRAM 64 KB
ROM 32 KB
TEXT MODES 40 x 24 / 80 x 32-28 / 84 x 64
GRAPHIC MODES Eight graphic modes, max : 672 x 512 (2 col), most used : 256 x 160 (16 col)
COLORS 256 (simultaneously in the mode 180 x 80)
SOUND 3 channels + 1 noise channel, 8 octaves
SIZE / WEIGHT 40 x 27 x 2,5 cm
I/O PORTS RGB Video / Audio output, Expansion port, Tape Interface, Joystick (2), Cardridge slot, RS432 (Serial/Net), TV output, Power input, Printer
OS IS-DOS, EXDOS
POWER SUPPLY External PSU

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TMR
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Re: The 8 Bit tech comparison Thread

Post by TMR » Wed Aug 15, 2007 9:45 am

GarryG wrote:I thought most surveys and stuff on this type of thing agreed that the Enterprise was generally the best 8Bit machine from a technical viewpoint?
Well we're really just making comparisons rather than arguing which is best, but the Amstrad CPC Plus series and the MSX Turbo R are both well into the same league (and what a shame the C64DX was never finished...) so it's more a case of how those specs are implemented that matters rather than just numbers. i can't do that comparison, i've only ever seen a CPC Plus on a couple of occasions and have never used the other two.

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GarryG
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Post by GarryG » Wed Aug 15, 2007 10:19 am

OK, then based on comparisons of how the available technologie were used for the machines I had:

I'd say the C64 was the best for sound, technicaly probably the best chip and it was well used in a lot of games.
The c64 also probably had the best implementation of hardware sprites at the time of it's release, the important fact is that these lent themselvs well to certain types of games and were put to good use in a lot of cases.

The Spectrum was the best for hi-res graphics (yes it didn't have the colour capability of the other machines, but the graphical implementation of some games was astounding for this machine, given the limitations), and you always got a rock solid picture from a spectrm, at lest I did, where the C64 and Atari screen display could sometimes be a bit ropey through the RF signal)

The Atari 8bit was best for colour. Ok, I know some people are going to argue the technicalites with this, but when you see two of the same games running on the origonal equipment side by side on the same TV the coloures on the Atari version inveriably look better than the C64, not 'bashing' here, and you can't see this by looking at stuff on enulators.
And I'm not talking about the available modes, or resolutions or numbers of colours available on screen etc. Just purely on how it looks when you see two almost identical games running!
I don't think this is subjective because it should be noticable to everybody. You may be able to 'emulate' the atari screen by bumping up the colour, contrast, brightness for the C64 screen, but in like for like, the colours seem more intense and defined on the Atari.


All the above is strictly my Opinion only, ;)

OOPs, didn't mean to edit this :oops:
Last edited by GarryG on Fri Aug 24, 2007 8:22 am, edited 3 times in total.

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Post by markopoloman » Wed Aug 15, 2007 10:27 am

Hmmm, the comparison I am doing at the mo involves the C64, Amstrad, Speccy and Atari.

The Atari Graphics and most notably the colours are just plane awful! But then thats down to my opinion (and I would imagine a really CRAP programmer) :lol:

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