Date   : Sun, 04 Jan 2015 20:53:10 +0000
From   : jgh@... (J.G.Harston)
Subject: Serial console active at power-on

Daniel Beardsmore wrote:
> Holding ctrl+shift at power on causes a different behaviour to
> unplugging the keyboard. The former pauses the OS in MODE 7 before
> POST completes, while the latter hangs the machine in what seems to be
> MODE 0 or 3 (the cursor is clearly a lot narrower).

It's in MODE 0. With no keyboard plugged in the MOS "sees" every key 
pressed *including* every keyboard link made. Which means it sees the 
startup settings (OSBYTE 255) set to zero (MODE 0, Boot, DiskSpeed=0) 
*and* sees Ctrl and Shift being pressed. So, it starts up in MODE 0, 
then tries to display <newline>"BBC Computer 32K"<newline> and sits 
there on the first <newline> waiting for Ctrl+Shift to be unpressed. 
Exactly the same as if you manually hold Ctrl+Shift down.

> Holding down as many keys as I can doesn't cause POST to hang

You can - press Ctrl and Shift. That's what's the MOS is seeing - Ctrl 
and Shift being held down, permanently.

(Technically, you're not stopping the POST, that has completed ages ago 
before the startup gets as far as attempting to display the startup 
banner.)

-- 
J.G.Harston - jgh@...      - mdfs.net/jgh