Date: Tue, 19 Aug 2003 15:30:36 -0700 From: John Niven <[EMAIL PROTECTED]>
Of course, with OS 6 I can't use my 128MB ram, so I setup a RAMdisk.
32 bit addressing was not introduced until System 7.x? I had never noticed that before. I guess by the time I had something that could take more than 4 MB of RAM, 7.x had been out for quite a while.
Now this got me thinking - why can I only use 8MB of program RAM? It's called 24 bit addressing, but 2 to the power of 24 is a 16M address space.
So what holds OS 6 back to 8MB, and why isn't that referred to as 23bit addressing? If there is no other way around it I may go back to 8MB of RAM so the machine boots faster.
The available addresses, the address space, is the entire universe to the computer's CPU. So even though the CPU may be using 24 wires for addressing (e.g. original 68000, Plus, SE) it cannot address 2 ^ 24 = 16 MB of RAM because some of those addresses must be used for other things besides RAM. Similarly, a machine in 32 bit mode cannot address 4 GB of RAM, because some of the addresses are devoted to other things.
So some of the addresses are allocated for the ROM, and some of the addresses are allocated for the various input and output ports on the computer. And usually the addresses are allocated in large blocks that are even powers of 2. On machines with slots, such as the Mac II, chunks of address space are allocated to each expansion slot, as a space where the card in the slot and the CPU can exchange information.
The address map is usually built into the firmware (ROM code) of the machine. For the machines which support both 24 bit and 32 bit addressing, they have two address maps. One is active in one mode and the other active in the other mode.
The address maps for all the early machines (through the IIfx, I think) are published in "The Guide to the Macintosh Family Hardware" which was one of those Addison Wesley books that came out along with the "Inside Macintosh" volumes.
Regarding the machines that are limited to 10 MB or similar, even though they support 32 bit addressing, that was just marketing weaniness on Apple's part. Unfortunately, they got the best of the engineers. There was some discussion on this list of trying to undo the limit at one point, but I think noone wanted to try to disassemble the ROM code and decipher the cause of the limitation.
Jeff Walther
-- Vintage Macs is sponsored by <http://lowendmac.com/> and...
Small Dog Electronics http://www.smalldog.com | Enter To Win A | -- Canon PowerShot Digital Cameras start at $299 | Free iBook! |
Support Low End Mac <http://lowendmac.com/lists/support.html>
Vintage Macs list info: <http://lowendmac.com/lists/vintagemacs.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive: <http://www.mail-archive.com/vintage.macs%40mail.maclaunch.com/>
Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
