On Sat, 30 Apr 2005, Eric Smith wrote:

> I wrote:
> > You're claiming that a 256 Kbyte RX01 floppy disk doesn't have
> > sufficient capacity to store 48 Kbytes of microcode?

> Make that 96 Kbytes; I forgot to multiply by two since there are two
> JCS modules. Though 96 Kbytes is the upper bound, and it seems likely
> that the standard load is smaller. Once the basic VAX instruction set
> is available, the system can boot, and can pull in optional microcode
> from elsewhere.

I'm probably getting things in the wrong order here, since I haven't read
the thing Eric quotes yet...

First and foremost, the RX01 must hold the VMB, which is quite large.
The version I have here is 85 blocks, or about 42 Kbyte.
Second, you never know what/if you have any other storage device, so you
cannot have parts of the microcode located on different places. All have
to be there right at the start. The PDP-11 reading in VMB can't even
access any other devices. The PDP-11 pulls in VMB, which executes in the
VAX, and at that time, the whole VAX must be there.
And then you need the boot scripts, everyone takes a K or two. You usually
might find around 10 boot scripts on a disk.

I don't know for sure how the PDP-11 on the 11/78x worked. Does it have
all software in rom, or does it boot from the RX01 as well? On the 86x0,
the PDP-11 is running a special version of RT-11 from the RL02.

Then we have the basic diagnostic monitor, which usually is on the disk as
well, even though much of the fancier diagnostics might be on another
set of RX01.

To finally take some sizes of microcode files from the 8650...
KA8600.BPN 382

Total is 466 blocks, or 233 Kbytes. Now, this is a 8650, so granted it is
probably a larger set of microcode than the 11/78x, but it still gives you
a ballpark figure.
Most of the microcode is the VAX cpu stuff, in KA8600.BPN.

So, where did that 48 or 96 K come from?


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|| on a psychedelic trip
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