Atari St Blitter

[Algernon Alcala]

LaterST's had a socket space for a BLiTER (BLock Image Transfer, or BLIT) chip but was never fitted. There are a few theories as to why this was. Ranging from, the Blitter wasn't ready when the STFM went to market, or Atari were going to offer it as a upgrade later on, but never did. It was probably it wasn't ready, or Atari intentionally held back the Blitter until the STE was released. While the STE had stereo sound and a better sound system overall, it also offered a larger colour palette and the next TOS version 1.62, There wasn't any real "power" increase in the hardware. IMHO Atari held back the blitter for the STE to make it a better machine overall than the ST series. The Blitter of course giving some useful features to speed up some graphic related operations. The STE without the Blitter would seem a bit of a let down upgrade wise for those who had brought a ST, at least it would in my opinion :) If anyone knows the "real" story about the blitter in the STFM then please let me know.

What actually is the Blitter for ? The BLiTTER is just the Bit-Blt (BLiT) algorithm as specified by Newman and Sproul ("Principles of Interactive Computer Graphics", McGraw-Hill, 1979, Chapter 18). It is meant to copy graphical data, organized in bit maps, and manipulate those by applying masks, halftones and logically combine source and destination. The Atari STE BLiTTER is a slightly more advanced version of this set of algorithms since it has direct memory access and does not rely on an external bus master to feed it data. (ref. _FAQ.TXT)

The main problem with upgrading the STFM is the lack of the Blitter chip itself and the PLCC socket. Generally new blitters can be obtained buy often at a high price along with the sockets often around $50. Secondhand/used ones are about much cheaper, but the sockets are generally old now and I wouldn't class them as reliable anymore. Generally the only source for the PLCC sockets is old motherboards. This is due to the fact Atari (also some other manufactures at the time) used a odd "offset pin" aligned PLCC socket. These offset pin sockets are next to impossible to find and if found, tend to be a high price compared to modern PLCC sockets.

As you can see on the above images, original socket (left) the pins are not aligned like they are on newer sockets (right). So Atari users (among other retro hardware) you cannot simply buy a new PLCC 68 socket and expect it to fit. This has been catching people out for many years :)

As can be seen the adapter is very small and fits neatly under the modern PLCC socket and solders into the motherboard. The overall height increase is typically about 2-5mm depending on how it is assembled.

If you have a connection, that is the TOP of the PCB (which is where the socket will sit on top of). If you do not have a connection then your PCB is upside down. So flip it over and re-test with meter.

First of all the Funky PCB slots into the normal PLCC socket pins as shown above. What must be done next is to trim the pins so they are level with the PCB. Yes I really did just say that. Some people may break into a cold sweat at the thought of not having pins pushing beyond the PCB by a few millimeter's. Though these holes are PCB VIA's and are tin plated "though" the hole. So what we do is solder as normal, but use the solder sparingly and with a good flux content. What will happen is the solder will be sucked into the VIA's (Capillary action in fancy talk)

So why are we cutting these pins I hear you cry ? Well firstly, the pins will get in the way of soldering the header pins, secondly, if the pins are not cut then they may short out on the motherboard itself as often there are multiple vias under the socket. Also on the Atari STFM, there is a height limit, which unless the whole adapter is kept as short as possible, it is unlikely to fit under the metal shielding, Thirdly, I can sit laughing at you as the pins go flying across the room as you are cutting them off, watch'ya eyes! Not sure if I mentioned actually soldering the pins yet, but you should probably go do that next :) Also watch that you do not block up any of the other holes on the PCB. If you managed to block some holes up (you did didn't you ?) Then I would suggest fluxed de-soldering braid to suck up the solder, Otherwise the header pins won't fit in the holes in the next step , and we wouldn't want that would we.

I recommend using long header pins. While shorter pins can be used, it can be a bit more tricky to cut them as explained later.. The pins strips (at least the ones I have) come in strips of 16, so they are broken into 4x8 and 4x9pin strips. The 8 pin ones are placed first as shown in the image on the right. BUT, I do recommend using a second Funky adapter PCB to hold the pins while they are being soldered. Otherwise if the pins are not perfectly straight you may well have insertion problems.

No real project is without PVC bodge tape, so here is its use in the assembly. Firstly just stops the "top" PCB from dropping down while the pins are being soldered into place. Secondly, squishing the tape roll makes a useful holder to solder the pins. Dual use PVC bodge tape- awesome :o) I could have gone one better and used 2 different colours of bodge tape, but that would have involved getting up off my chair, and the floor looks a long way down from where I am sat :o)

In goes the 9 pin header pins. Simply pushed in though the top PCB and then soldered in at the bottom. I STRONGLY recommend (notice the caps and the bold text there! I could have underlined it, but I didn't want to over do it!) you double check there are no shorts on any of the pins at this point.

The spare Funky Adapter PCB is just pushed down out of the way as shown on the left most image. At some point ( I think it was yesterday) I mentioned using smaller header pins. If you use the smaller height pins, then you will have to cut the pins as close to the black plastic part as possible. Otherwise, the pins will not be long enough to solder into the motherboard. So they have to be done neat and accurately. I use the longer pins as you can pretty much hack them at any length since they will have to be cut again once soldered into the motherboard anyway.

At this point remove the "spare PCB" and you are ready to solder the socket into the motherboard. It is important to keep the socket as low as possible otherwise the metal shielding will not fit above it.

What I suggest is cut a small piece of card (cereal box is ideal as its thin) and place it under the socket PCB so it does not short out on the motherboard via's. This way the socket can be pushed down as far as possible without risking shorting on the motherboard.

A blitter is a circuit, sometimes as a coprocessor or a logic block on a microprocessor, dedicated to the rapid movement and modification of data within a computer's memory. A blitter can copy large quantities of data from one memory area to another relatively quickly, and in parallel with the CPU, while freeing up the CPU's more complex capabilities for other operations. A typical use for a blitter is the movement of a bitmap, such as windows and icons in a graphical user interface or images and backgrounds in a 2D video game. The name comes from the bit blit operation of the 1973 Xerox Alto,[1] which stands for bit-block transfer.[2] A blit operation is more than a memory copy, because it can involve data that's not byte aligned (hence the bit in bit blit), handling transparent pixels (pixels which should not overwrite the destination), and various ways of combining the source and destination data.

In computers without hardware accelerated raster graphics, which includes most 1970s and 1980s home computers and IBM PC compatibles through the mid-1990s, the frame buffer is commonly stored in CPU-accessible memory. Drawing is accomplished by updating the frame buffer via software. For basic graphics routines, like compositing a smaller image into a larger one (such as for a video game) or drawing a filled rectangle, large amounts of memory need to be manipulated, and many cycles are spent fetching and decoding short loops of load/store instructions. For CPUs without caches, the bus requirement for instructions is as significant as data. To reduce the size of the frame buffer, a single byte may not necessarily correspond to a pixel, but contain 8 single-bit pixels, 4 two-bit pixels, or a pair of 4-bit pixels. Manipulating packed pixels requires extra shifting and masking operations on the CPU.

Blitters were developed to offload repetitive tasks of copying data or filling blocks of memory faster than possible by the CPU. This can be done in parallel with the CPU and also handle special cases which would be significantly slower if coded by hand, such as skipping over pixels marked as transparent or handling data that isn't byte-aligned.

1973: The Xerox Alto, where the term bit blit originated, has a bit block transfer instruction implemented in microcode, making it much faster than the same operation written on the CPU.[1] The microcode was implemented by Dan Ingalls.[1]

1982: In addition to drawing shape primitives, the NEC μPD7220 video display processor can transfer rectangular bitmaps to display memory via direct memory access and fill rectangular portions of the screen.[3][4]

1982: The Robotron: 2084 arcade video game from Williams Electronics includes two blitter chips which allow the game to have up to 80 simultaneously moving objects.[5] Performance was measured at roughly 910 KB/second.[5] The blitter operates on 4-bit (16 color) pixels where color 0 is transparent, allowing for non-rectangular shapes.[6] Williams used the same hardware in other games from the time period, including Sinistar and Joust.[6]

1984: The MS-DOS compatible Mindset personal computer contains a custom VLSI chip to move rectangular sections of a bitmap. The hardware handles transparency and eight modes for combining the source and destination data.[7] The Mindset was claimed to have graphics up to 50x faster than IBM PC compatibles of the time,[8] but the system was not successful.

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