Posted Tue Mar 9, 2010 10:34 AM
Haha, well this thread is the end-all be-all of information on the Internet. :-)
VDP == Video Display Processor. It is really a generic term, but not used much any more. Now days we just say Radeon or gForce. In the TI, the VDP is the TMS9918A which is a single chip that takes care of displaying the video image. The 9918A also has its own RAM, typically called VDP RAM. The CPU can get access to the VDP RAM but it has to do so by going *through* the VDP, which can be a slow process since the VDP is almost always busy drawing the screen. The 9918A supports a few graphic "modes" as well as the 32 sprites. However, there is a limitation on the number of sprites the VDP can display on a single horizontal scan line (due to the amount of time the VDP has to process a horizontal scan), so after 4 sprites are displayed on a given scan line, the VDP stops drawing sprites on that line. This is the "4 sprite" limitation you see us referring to. The VDP does *not* automatically move the sprites, and the auto-sprite movement you might read about on the TI is done by the one and only ISR (interrupt service routine) in the console. The ISR is triggered by the VDP vsync 60 times per second, and it can be disabled (which is usually done for assembly games.)
BLWP == Branch and Load Workspace Pointer. It is a machine instruction (assembly language mnemonic) that was primarily designed for doing OS context switching since the TMS9900 was most used in TI's mini computers. It is one of a few ways to do function calling in assembly, and definitely the slowest way to do functions. It has a few advantages though that depending on the circumstances can be worth it.
You can read about the gritty details in the EA (Editor Assembler) manual or the TMS9900 Datasheet. I'm pretty sure both are available on the WHTECH FTP site, or I think there is a torrent set up too? Anyone? The EA manual was the "big book" that came with the Editor/Assembler package and is the de-facto reference for the 99/4A (although not complete.) The PDF I have is a scan of every page and weighs (virtually speaking) as much as the real book. The 9900 Data Manual is the datasheet that TI put out for engineers to build computers with the 9900 CPU and has all the low down details about instruction execution, timing, internal operation, interfacing, etc. I use both quite a bit, and the especially the data manual when working on my emulator.
Hope this helps some.