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Sunday, August 23, 2009
& Very Old Things - Part 1
Bring back any painful memories?
So, a couple days I had one of those evil, perverted urges I'm so notorious for. Specifically, I felt like doing a bit of hacking of Blaster Master. For those not familiar with it, it was an NES game published 21 years ago. A good one, in fact. But before I talk a bit about anything I did resembling reverse-engineering, let me explain the NES hardware to the best of my knowledge (which consists of what I saw in my hacking, and a bit of stuff I looked up online).
The main CPU of the NES is a modified 6502, the CPU used in various Commodore and Apple computers (not that I ever used one of those). The 6502 is a primitive CISC CPU, and in fact somewhat resembles the x86 (though more primitive). It has 3 8-bit registers: a numeric accumulator (A) and 2 index registers (X and Y). Operations generally operate on the value of A and a memory location or literal, and write the result back to A.
It has a stack, though from what I saw in Blaster Master it seems to be relatively rarely used, used mainly to save registers (though even that was pretty rare). Specifically, I didn't see much in the way of stack frames holding local variables. Functions that require local variables tended to have (dedicated) memory ranges allocated to them to be used as private buffers. For example, one address I initially thought was the boss life location actually turned out to be in a buffer region used by the function while updating objects in the object list. This kind of surprised me, as the NES has only 2 KB of RAM; I imagine this must be forced by the architecture, e.g. a lack of instructions to access the stack at a specific offset.
As for the graphics system. If you ever messed around with text mode and text graphics in DOS, the NES graphics system is similar. At the lowest level, graphics are stored as blocks called patterns. Each pattern is 8x8 pixels, 2 bits each (total of 4 colors). The NES has enough video memory for 512 patterns at a time, stored in 2 pattern tables. What exactly it does with these patterns depends. The NES has 3 graphics layers: 1 for background and 2 for sprites (1 in front of and 1 behind the background layer).
deconstructulator provides an excellent illustration for most of the things discussed here. It shows the pattern tables (note that the top half is the sprite pattern table and the bottom half is the background pattern table) and sprites along with a playable version of Super Mario Bros.; although it does not show the name/attribute tables. Refer to that from here onward.
The background layer (illustration below) is very much like the screen buffer in DOS text mode; it's a buffer of screens composed of 32x30 patterns each (indices into the pattern table), called name tables (lost in translation, perhaps?); all patterns in the background must come from the same pattern table. The NES supports up to 4 screen buffers in this way, though it only contains enough memory for 2 of them (if the cartridge doesn't supply additional memory, the game can simply set the NES to mirror the 2 screens).
However, so far each pixel only has 2 bits of color (the bits in the pattern table). An additional table, called the attribute table, supplies an additional 2 bits to the name tables, increasing the total to 4 bits, or 16 colors (though each tile on screen still has only 4 colors within it). However, the attribute table does not contain a value for every tile. Rather, it only contains 1 two-bit entry for each block of 2x2 entries in the name table (so 16x16 blocks composed of 4 patterns). This is the reason 16x16 blocks appear everywhere in NES games - that's the smallest block that can be fully independently controlled.
The background position is specified independently of the sprites. A horizontal and vertical scroll position is specified by pixel for the background, which indicates where the graphics chip should start drawing the background on screen. This is how smooth scrolling works, although implementing this can be quite a trick, as you have to essentially use the name tables as a ring buffer (even more tricky if you support scrolling in both dimensions).
A shot of the side-scrolling portion of Blaster Master (top) and the corresponding, merged name/attribute tables (bottom). The white lines indicate where the top left of the screen currently is (where the two lines cross). The visual discontinuities are caused by the way the game streams the background into the name/attribute tables on demand, filling in only what's on screen in the frame; they're stale data. The color glitches result when the current color in the attribute table for an entry does not match some of the stale patterns still in that name table for that entry (e.g. on the far left the second column of patterns is discolored because they are stale and the attribute entry now corresponds to the first column of patterns, which is on screen).
The NES supports 64 sprites, each either 1 or 2 patterns in size (8x8 and 8x16, respectively), depending on whether a global flag is set. Analogous to the attribute table, each sprite contains the 2 high bits of color (though still that allows only 4 colors per sprite). Other info each sprite possesses includes position (obviously), mirroring parameters, and whether the sprite is in the foreground or background. However, even if you can have 64 sprites, the NES only supports 8 sprites per scan line; if there is more than that, only the 8 with the highest priority are drawn (smart games can cycle through their sprites each frame, but that results in flicker).
There are, however, some tricks that can be done to circumvent some of the innate limitations (e.g. 16 colors, 16x16 blocks, 64 sprites, etc.). These often involve clever but perverse tricks related to swapping out the various things such as scroll offset or sprite data while drawing is underway. One common use of this is to draw part of the screen (the gameplay) with scrolling, while a portion of the screen on top or bottom is fixed (the score area). This can also be used to draw more than 64 sprites per frame, though there's still a limitation of only 8 sprites per scan line.
Blaster Master and others frequently use the background as a grotesque way to draw very large sprites. In the above screen shot, the boss crab is actually the background, while the bubbles, player, and energy bars are sprites (the green areas of the boss are actually sprites, as well). The crab has a couple frames of animation; each of these is on a separate screen in the name table, and every frame the scrolling position is modified. This results in the crab moving around the screen and animating, as if it were a huge sprite.
Oh, and in case you were wondering, yes, this boss frequently exceeds the 8 sprites per scan line limit (that would be 4 bubbles), resulting in the game slowing down to draw them all.