30 Oct
2017
30 Oct
'17
12:42 p.m.
On 10/30/2017 04:18 AM, Eric Smith via cctalk wrote:
2. What a UTM does is simulate another machine using
only a general-purpose
machine. In fact, the UTM is arguably the most general-purpose machine ever
described. What IBM defined as emulation was use of extremely specialized
hardware and/or microcode (specifically, not the machine's general-purpose
microcode used for natively programming the host machine). If anyone else
did _that_ in a product before IBM, I'm very interested.
Well, "emulate" is a very fuzzy word, even in the context that you provide.
Taking as an example, some pre-S/360 hardware that's easily verified, is
the IBM 7094. It ran the instruction set of the 7090 but with a single
departure--the use of 7 index registers, selectable using a 3-bit
isntruction field. In the 7090, that same field selected any of three
registers--if you selected more than one, the contents were ORed
together before subtracting from the base address. The 7094 did have a
"7090 compatibility mode" that used only the first three index registers
in the same manner as the 7090.
Specific hardware than enabled one machine to act as another.
So, is that "7090 emulation" on a 7094?
--Chuck