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Why Lisa didn't use 8MHz 68000?

Gorgonops

Moderator
Staff member
Wow, the proposal actually exists. It's kind of amusing, re: the discussion about page faults in the other thread, that item #1 on the "Additions and Enhancements" list is:

The 68010 has been chosen because it supports instruction restart and fast move loops.

Anyway... Skimming through this the details of the "CPU board" are fascinating. The "video" section of it has a full 128K of RAM (48k of which are used for a single frame buffer), and that new sound generation hardware that leeches off the video cycles (just like the Mac) is integrated with it. Honestly, this architecture pretty much reads like they plopped, in whole, a juiced-up revision of the "little Mac" on top of the Lisa-related bits, with the major difference being that they sped everything up and gave the CPU a fast bus channel where other "stuff" can be hung without being strictly gated by video access timings. This impression is further reinforced by the fact that the CPU board also contains the the IWM, now directly driven, the same serial ports... etc.

(Even more telling, the docs seem to imply that initialization code and "stuff" that has reasons to bypass the MMU can also run directly from the video RAM allocation.)

I guess to put it another way, if we were going to use "Amiga-centric" language to describe this thing it basically comes across as the CPU board essentially being the "Chip RAM" portion of the design, and the "Whopper MMU" is tending the "Fast RAM".
 

sunder

Well-known member
Hi Tom, sadly no Lisa 3, a color Lisa, or the Whopper made it past a thought exercise phase, so it's all speculation at this point.

You are correct, technically you can point the video buffer on a Lisa anywhere within the physical 2MB RAM space, outside of the MMU. So it's not just two screens, but you could do up to 2MB/32KB or 64 screens (though most Lisas were limited to 1MB or 1.5MB, and a handful to just 512KB.)

Certainly if you did dedicated RAM for a frame buffer, then you're limited to whatever RAM you have available for that, so if it's just 64KB, it would be 2 screens max. It's true that the alternate screen was used by LisaBug, but it's not actually limited to just two (for those who aren't familiar with the Lisa's HW design.)

I'm not saying that the Lisa was beholden to Apple ][ ways of doing things either, but rather, "here's a quick hack, we can copy what the Apple ][ does for floppy access by creating a tiny dedicated Apple ][ inside the Lisa, and boom, we're done, we can ship it." I feel that was a wasteful way of doing things, but yeah, it did help it ship on time for sure, but at a much higher cost and complexity.

Sigh... I really wish there was a Whopper, or Lisa 3, or color Lisa. alternate universe/timelines and all that, it would have been amazeballz. :) And ofc, would have been nice if it had digital sound in/out too, and ethernet, and so on...
 

lisa2

Well-known member
I seems to me that when folks learn that the Lisa was clocked at 5Mhz vs. the Macintosh at 8Mhz it gives the impression that the experience of using a Lisa is almost 40% slower than using a Macintosh. This is not helped by reports that using the Lisa Office apps are dog slow.

My experience in using both the Lisa and Macintosh side-by-side does not support this. The Lisa may be clocked at lower speed, but it certainly does not "feel" almost 40% slower in use. I ran a benchmark application on the Lisa ( Speedometer 2.0 ), and it rates the Lisa at .73 the CPU rating of a Macintosh SE. This benchmark shows that there is more going here than raw clock speed. Compared to a floppy only Macintosh 128K or Fat Mac the Lisa with her 1 Meg of RAM and hard disk is very useable and on-par or better than the experince on an early Macintosh.

Rick
 

sunder

Well-known member
I mean, I wouldn't really use a Lisa to run Mac software anymore* so it's, uh, like comparing apples to oranges, if you'll excuse the pun. MacWorks may well be far more efficient at running a single application vs multiple ones.

However, I'd also point out that most of the time, all computers are unused. They sit there idling. The only time it feels slow is when it's booting, installing LOS, formatting a Widget/ProFile, or doing some intensive disk, or CPU operation. Scrolling on a Lisa with LOS is pretty snappy - well it depends on what had to be refreshed with QuickDraw and how complex that was... So context here is the always key.

If you're doing nothing and just wiggling the mouse, it's very responsive on a MacWorks as well as LOS. If you're swapping to disk, or fully recalculating a huge spreadsheet, well, that's going to be a lot slower. (And MacOS of that era did not do virtual memory.) Certainly copying lots of things off floppy is going to be a lot slower than from ProFile to ProFile.

I don't know if LOS does file system caching if there's free RAM around like modern Linux/BSD does, certainly there's no RAM disk, but if it did that would increase the speed of the machine. Certainly some of the disk structure is cached in RAM from the behavior I saw when working on LisaEm and disassembling parts of LOS. (And to be honest, I don't really know if classic MacOS does disk caching and how effective it is, I'm sure it's not as good as modern day Linux/BSD.)

So benchmarks might not be all that useful as using real apps when saying that the Lisa is slow or not vs newer Macs. I suppose if you fire up MacDraw and LisaDraw and use both, that would be a better benchmark to see which is faster.

* well that's a kind of lie as I do have one Lisa dedicated to just MacWorks, but for other reasons than running classic macos, in real life I just use mini-vMac for that kinda thing, and also 99% of the time use LisaEm than a real Lisa anyway and can throttle up the speed, and also the whole thing pretty much lives in RAM, including floppy and ProFile images due to our friend mmap(). Pretty much the only time I turn on real hardware these days is to repair it or test things that don't work on a real Lisa to figure out behavior. Even when the throttle is at 5MHz, it's a lot faster than a real Lisa due to most of it being in RAM (and certainly the High Level Emulation of disk access removes a lot of the transfer waits due to the protocols used.)

If anything the HQ3X video renderer with scaling is more CPU intensive than the 68000 emulation on this Linux laptop.
 
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