[COFF] [TUHS] Re: machine code translation, as mental architecture models

[John R Levine]

On Thu, 11 Jul 2024, Paul Winalski wrote:
> Yes, that was precisely my point, and thank you for stating it more clearly
> and concisely than I did.  The VAX MACRO compiler takes in VAX assembly
> source code, not binary VAX instructions.

Does anyone know how extensively they used the macro facilities?  You can 
write much higher level stuff as macros than as single instructions, which 
makes it a lot easier to do efficient translation.  For example, on OS/360 
you'd write a GET macro to retrieve the next record (or a pointer to it) 
from a file, which was a lot easier to figure out than the control blocks 
and subroutine calls the macros expanded into.

> were done completely in hardware--no microcode.  S/360 models 67 and down
> were microcoded.  The lowest end S/360, the model 25, was actually a 16-bit
> machine.

The /25 and /30 were 8 bits internally and as slow as you would expect, 
but were still full implementations of S/360.  IBM sold a lot of them. 
The /40 was 16 bits, /50 32 bits, and /65 64 bits.  The later /85 was 
roughly a /65 reimplemented in faster logic with a cache and 128 bit 
memory, making it as fast as the more expensive /91 for programs that 
didn't use a lot of floating point.  The /85 was microcoded and could 
emulate a 7094.

   All the S/360s from the 65 and down had microcoded emulators for
> 2nd generation IBM architectures such as the 1400.  The emulators usually
> ran faster than the real hardware.

They always ran faster, company policy.  That's why you needed a 360/65 to 
emulate a 7094.

> The emulators were there to accommodate data centers that had lost their
> source code for mission critical applications.

Partly that, more that the emulators allowed the customers to spread the 
conversion work out partly before they got the new machine, partly after. 
Remember that the new machine was faster and had better peripherals. 
Since it was built with more modern components it may well have been 
cheaper to rent.

> Knuth was right about multiple levels of emulation/interpretaton.

The 5100 was indeed a marvel, but it was a very slow one.

Regards,
John Levine, johnl at taugh.com, Taughannock Networks, Trumansburg NY
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