From mboxrd@z Thu Jan  1 00:00:00 1970
From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Date: Wed, 23 Dec 2015 01:59:57 -0500 (EST)
Subject: [TUHS] Does anybody recall how the TU10 bootstrap code actually
	operates?
Message-ID: <20151223065957.5ED5718C095@mercury.lcs.mit.edu>

    > From: Will Senn

    > 000777  HALT

That's actually "BR ."; the difference is important, since the CPU (IIRC)
doesn't honour DMA requests when it is halted, and DMA needs to be working for
the controller to read that first block (a secondary tape bootstrap) into
memory.

    > This seems like gobbledegook to me.

:-)

    > It moves the MTCMA (Magtape Current Memory Address) into R0, then it
    > moves the MTCMA into the MTBRC (Magtape Byte Record Count)

"The address of the MTCMA into", etc. Looking quickly at the programming spec
for the TM11 controllers, it wants a negative of the byte count to transfer in
this register; the address of the MTCMA just happens to also be a large enough
negative number to be usable as the (negative) size of the transfer request.
(The first record is probably shorter than that, but that doesn't matter.)

Note that this code could probably also have been written:

    MOV #172524, R0
    MOV R0, at R0

and been equally functional.

    > then it moves 60003 into the MTC (Magtape control register), which
    > causes a read operation with 800BPI 9 Channel density.

I'm too lazy to look at the programming spec for the details, but that sounds
right.

    > Am I misinterpreting the byte codes or is this some idiosyncratic way to
    > get the Magnetic tape to rewind or something (the TM11 has a control
    > function to rewind, so it seems unlikely that this is the case

No, it's just the shortest possible program to read the first block off the
tape.

It depends on i) the operator having manually set the tape to the right point
(the start of the tape), so that it's the first block that gets read, and ii)
the fact that the reset performed by hitting the 'Start' key on the CPU clears
the TM11 registers, including the Current Memory Address register, so the
block that's read is read into memory location zero.

Hence the direction to 'once the tape has stopped moving, re-start the CPU at
0'.

	Noel