Friday 6 January 2023

Comparing the Graphics Capabilities of the MEGA65 and Amiga

This post is just a quick summary of some replies I gave on Facebook recently, when people were asking questions that made it clear that they had the mistaken belief that the MEGA65's graphic system is not powerful.  This is a repeating theme I have noticed around the internet from time to time. Perhaps it is because a lot of people do things on the MEGA65 that are not graphically taxing, like playing Infocom text adventures. Anyway, someone posted a question that was along the lines of: "The MEGA65's graphics seem pretty underwhelming", and with the Amiga being given as a point of comparison.  So I wrote the following explanation, that I figure will be useful for folks to refer to when trying to understand the relative graphics power of the two systems. 

@retrocogs' work-in-progress game for the MEGA65

(Before diving into that, it's worth mentioning that the MEGA65 is undoubtably ahead of the Amiga on the sound front: It has 4x DMA driven audio channels like the Amiga has, but they can drive sample rates up to 2MHz, and each channel is fully stereo pannable, and support not just 8 bit samples, but also 4 bit packed, and 16 bit samples.  This is in addition to the MEGA65's 4 SIDs. The MEGA65's floppy controller is also more versatile, able to read any DD or HD formatted disks, and has hardware acceleration not just for MFM encoding, but also for RLL encoding, which can give a capacity boost of another 50% or so. Finally, it doesn't actually matter which is better than the other. Both the MEGA65 and the Amiga series are very enjoyable machines, regardless of their theoretical capabilities.)

In the Amiga, the blitter was normally used on the Amiga to blit images onto one or more bitplanes, to give the effect of having more and/or larger sprites (even though we both know that they aren't sprites, but rather are "blitter objects" or bobs.) The Amiga's use of blitter objects was limited by the number of cycles available to the blitter per frame, and the fact that to move a bob, you had to first undraw it, and then re-draw it. If you have a bunch of potentially overlapping bobs, you may have to undraw many, just to be able to move the one you want, and then redraw them all again. And the greater the bit-depth of the bobs, the greater the cycle cost, because multiple bitplanes had to be undrawn and redrawn. And you had to do it all during VBLANK, or double buffer the display to avoid tearing.
Now a bit of a journey that will help you understand the MEGA65's graphic systems, and it's capabilities:
The C65 inherited the Amiga idea of bitplanes, but they are greatly hobbled, mostly because you can't scroll them in hardware like on the Amiga, and the very limited "chip RAM" of the C65 meant that it was a recipe for disappointment. However, it did get 8 bitplanes, and thus a palette size of 256 colours, with 4 bits per channel giving 4,096 colours -- just like the ECS Amiga, but with the bitplane count of the (at the time) not-yet-existing AGA Amigas.
So while the MEGA65 implements C65 bitplanes for backwards compatibility, they have not been further improved. Instead, a much more C64-inspired approach was taken, along with some general underlying improvments.
First, the VIC-IV in the MEGA65 has access to the chip RAM at 81MHz, i.e., it can suck 81 MB of data per second. For PAL, divide that by 50 and you get 1.62MB of data that can be fetch per frame. The colour palette registers are on a separate bus, and so don't subtract from that, as is the colour RAM for describing the colour and attributes of each character or 8x8 cell of bitmap data. So we really have 1.62MB per frame. On NTSC with 60Hz its a little lower, at 1.35MB. But either way, that's still a lot of bytes per frame. The MEGA65 supports "overscan" modes, i.e., 720x576 for PAL and 720x480 for NTSC. So if we multiply those out the PAL and NTSC frames contain 415 K and 346 K pixels per frame. This means the VIC-IV has about 3.9 bytes per pixel of bus transfer available to it, in both PAL and NTSC (because the extra frames of NTSC are essentially exactly offset by the reduced vertical resolution).
Compare this to the C64 that had 0.125 bytes per pixel (8MHz pixel clock and 1MHz CPU). This is why the bitmap mode of the C64 was monochrome if you wanted full horizontal resolution (1 bit per pixel = 1 byte per 1MHz CPU cycle), and had to halve the horizontal resolution to get 2 bits per pixel for multi-colour mode. The Amiga 500 with it's 3.5MHz 16-bit bus and half the vertical resolution of the MEGA65 had typically ~3.5MB/sec for video memory accesses, thus giving it about 0.875 bytes per pixel of total bandwidth (there are some simplifications I am making here, but the general result holds). This is why the Amiga was capable of 64 colour modes, as on average, those require 6 bits per pixel, which was within its bandwidth limit of about 7 bits per pixel. It's also why higher resolutions on the Amiga had lower maximum numbers of active bitplanes.
The MEGA65 with its 81MB/sec dedicated video bandwidth has ~31x more bandwidth per pixel than the C64 had, and better than 11x than the Amiga 500.

The Amiga 1200 essentially quadrupled the video bandwidth compared with the Amiga 500, giving a video memory bandwidth of about 14MB/sec, or about 3.5 bytes per pixel. That's still below the 3.9 bytes per pixel of the MEGA65, but you get the idea that the video system of the MEGA65 has the potential to be at least as powerful as an AGA Amiga. But because of the design decisions we made for the VIC-IV, the result is actually more powerful for several reasons:
1. Bitplanes eat a lot of RAM if you have repetitious content or lots of empty bits on the screen, e.g., for a typical platformer for top or side view shooter. Also drawing such data is relatively time-consuming with bitplanes, because you have to write all those pixels whenever you need to redraw the display (eg when you hit the limit of your hardware scrolling region in the bitplane). Arcade systems and many game consoles used a different approach: Layered tiled displays, effectively multiple layers of what we would call text mode on the C64. The MEGA65 also uses this approach.
2. The VIC-IV adds two new text/bitmap display types in addition to the 1-bit per pixel mono/hires and the 2-bits per pixel multi-colour/lores types. The new ones are: Nybl Colour Mode (NCM), which uses 4 bits per pixel, thus allowing a character to have each pixel selected from a 16 entry long slice of the 256-colour palette (which has also been upgraded to 8-bit colour depth for millions of colours instead of the Amiga 500's 4,096 colours). Full Colour Mode (FCM), which uses 8 bits per pixel, thus allowing a character to have each pixel freely selected from the 256-colour palette. NCM and FCM both use 64 bytes to describe a character, instead of 8 bytes. As NCM uses only 4 bits per pixel, the result is that NCM chars are 16x8. FCM chars stay 8x8. NCM and FCM can be used to very easily and efficiently (both in memory and computation) implement map-based displays, as for shooters and platformers. FCM and NCM effectively require a little over 1 byte per pixel, thus leaving the bulk of the memory bandwidth free.
3. Because there is still plenty of bandwidth available, even with FCM graphics, the VIC-IV includes another improvement: A double-buffered raster buffer of pixel data that is rendered in one raster, and then displayed the next. Special character codes can be setup that move the current position within the raster buffer, effectively allowing the "over stamping" of characters (which can be mono, multi-colour, NCM or FCM). This removes the need for bitplanes to achieve "play fields". It also means that each playfield is effectively as powerful as all 8 bitplanes on an AGA Amiga. This buffer is called the Raster Rewrite Buffer (RRB), that you had heard about Simple in design, but extremely powerful in effect.
4. If you want to have even more layers with the RRB, you can switch to half vertical resolution (i.e., the same vertical resolution as a C64 or non-interlaced Amiga), to double the "raster time" for the RRB. This can be used to create many layers of graphics, well beyond what an AGA Amiga could manage. Remember that none of this takes a single cycle from the CPU -- The CPU retains its full 40.5MB/sec bandwidth, regardless of what the VIC-IV is up to, because the chip RAM is dual-ported.
5. You probably notices that FCM is effectively a "chunky" mode. If you arrange the screen with columns of ascending character numbers you get a layout that is effectively a chunky bitmap mode, and the pixel address computations are very simple. This gives the MEGA65 a further significant advantage over the Amigas. It was very easy to write a full 320x200 256 colour wolfenstein 3D engine with >10fps, without optimising it very much at all. This is partly due to another trick the MEGA65 has up its sleeve:
6. The DMA controller in the MEGA65 can perform many of the functions of the blitter in the Amiga, plus some extras. One of those is that you can tell it your FCM chunky screen layout geometry, and it can then fill, copy and/or scale arbitrary angled lines in hardware. This can be used to draw lines at 81Mpixels / second -- enough to fill the display with lines every single frame, plus some. It can also be used to copy textures at 40.5Mpixels / second, including scaling them. This means that for the wolfenstein 3D engine that I made for the MEGA65, the actual drawing of the display is the fast part, requiring maybe 1/4 of the available raster time each frame. The slow part is the logic to work out which surface to show, because I just wrote that in C using the robust but not particularly optimising CC65 compiler. Even with that, it obtains double-digit frames per second. With optimisation, full frame rate at 50 or 60Hz is very likely possible.
7. The sprites in the VIC-IV have also been substantially updated, now featuring upto 64 pixel horizontal resolution and the VIC-II's mono and multi-colour modes, plus NCM, giving 15-colour sprites upto 16 pixels wide. The sprites can also be upto 255 pixels tall. They also have an alpha transparency mode, and a horizontal tiling mode, and be switched between hires and lores (i.e., expanded sprites) as on the C64, but also into super-hires, where the sprites are half their normal dimensions, but with double the resolution (ie the opposite of "expanded sprites" on the C64).
8. The VIC-IV has four hardware palette banks of 256 colours each. NCM and FCM characters can select from two of those palettes on a character by character basis. Which 2 of the four palettes are in use at any point in time can also be changed via a palette selection register, thus making it very easy to have 1K colours on screen at any point in time, without having to resort to copying palette entries around the place. But with the spare palette banks and 40.5MB/sec DMA copying, it is also possible to reload the palette registers multiple times per frame, to yield many more unique colours on the screen at once. This is more flexible than HAM, and doesn't produce any visual artifacts unlike HAM.
There are some other tricks up the MEGA65's sleeves, but hopefully that gives you a good introduction as to many of the improvments, and why the MEGA65 is at least a match for AGA Amigas in terms of graphics, and in other areas well ahead of it.

1 comment:

  1. With regard to sound, I WILL get that OPL chip working eventually. ;-)