[TUHS] Harvard and Von Neumann Architectures and Unix

[Bakul Shah]

> On Nov 25, 2017, at 6:24 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
> 
>> From: Doug McIlroy
> 
>> Optimal code for bitblt (raster block transfers) in the Blit
> 
> Interesting case. I'm not familiar with BitBLT codes, do they actually modify
> the existing program, or rather do they build small custom ones? Only the
> former is what I was thinking of.

This was the only way we could handle four 9600 baud serial connections
on a 5.5Mhz 68000 processor. Back then the UART we used only buffered
one character (also the case with the old 16450) - so one interrupt per char.
And we "discovered" devices at runtime. So Yost wrote some code to hook
up customized interrupt handlers that stuffed a larger char buffer specific
to each UART. Essentially "partially evaluated" interrupt handler code to
minimize memory accesses. A separate s/w interrupt was used to service
the larger buffers.

This was done at boot time but another instance of self-modifying code
(that is not a virus) at runtime was by the old gcc compiler - don't know
what it does now - but pointer to a GNU C's lexically nested function was
actually a piece of code that was constructed at runtime with hardwired
static link in it (recall that a nested function can reference variables from
its lexical environment but normal C function pointers are represented as
a single pointer).