Re: address architecture, not interactive use, The Design of Design

Thomas Koenig wrote:

MitchAlsup1 <mitchalsup@aol.com> schrieb:

Thomas Koenig wrote:
>

John Levine <johnl@taugh.com> schrieb:

>

Brooks said it was ugly that some instructions (RX) had both base and
index registers while others (SS) only had base registers, which I
expect made it even harder to do what you suggested.

>

Depending on base registers for both data and branches was one
of the ideas that did not age well, I think.  We have since
seen in the RISC machines that having a stack implemented via
a register, with possibly a frame pointer, a global offset and
larger offsets (16 bits) works well, and we know how to generate
position-independent code.

>
Position independent data is still difficult, though.

Touché.  Data is not the problem, but pointers to date (such as
addresses of arguments) is...

Position independent external data requires a load of the base of the
region the data sits, and a second memory access to do something with
the data. You CAN use this base register multiple times--IFF you can
figure out matching regions external data resides and share use of the
base register.
Modern systems put these bases in GOT.
This is a hangover from the days where one built fully resolved object
modules. ASLR is a driving force for a "better solution" {whatever it
ends up being.}

So, having a base register added to all addresses of user code
would definitely have been a better choice.