Msg: 120 *Conference*
06-24-91 23:59:46
From: ED DAHLGREN
To : TED SWIFT
Subj: REPLY TO MSG #119 (REPLY RE: M100 EXPANSION)
Ted -- You're right that with 16 bits you can only address 64k of memory -- directly anyway. But consider another popular 8-bit processor, the 6809. It too uses only 16 address bits and has 32k of ROM, yet Tandy is currently selling its system built around this chip for $99, and it comes with 128k of RAM standard, and Tandy and several others sell 512k memory upgrades. Three or four companies sell 1 MEG RAM packages for it .... So it's not the memory that you can directly address that really matters, it's how you manage that space. With Tandy's 6809 (68B09E, to be exact), the 32k ROM is read into RAM and then drops out. What you choose to do with that space then is up to you (and your software). With their 8085 the only choice they give you for hex addresses 0000-7FFF is whether you use the on-board ROM or the option ROM. Then, on the circuit board holding the 6809, they put a Memory Management Unit that swaps 8k pieces (pages) of RAM into and out of the space that can be directly addressed. This is done for you automatically so that the entire RAM appears to be contiguous -- that is, you don't have to break your data or programs into chunks and store parts here and parts there. I have two of these systems, each with twin 360k disk drives attached. For one of them I have an attachment for the system bus that will connect to an industry-standard hard drive when I buy one. All of these drives run from the system bus, leaving the serial port free for other stuff. Sometimes I use it for my printer, which is switchable between parallel and serial operation, sometimes I use it to control my Yamaha piano and syntehsizer setup (MIDI standard). For those who insist upon a parallel port, one is built into the same box that houses the hard-drive interface, as is an additional 8-bit interface that I plan to attach an EPROM burner to. I apologize (a little bit) for sounding like I'm on a soapbox. I am, of course! (Insert smiley-face here.) My point about the serial port and system bus is the same as my point about 16-bit memory addressing -- it's what you DO with it that counts. In the case of the internal hardware, there are tradeoffs of course. In order to stuff in the Memory Management Unit plus a driver for 16-color 400x200 display (not included!) and a few other odds and ends, they made their 6809 box, oh, 3 or 4 times bigger than their 8085 box. But, that's the nature of the computer game! -- Ed