From: anton@mips.complang.tuwien.ac.at
Krishna Myneni writes:
>The intent of the stack array access was to avoid stack pointer
>arithmetic altogether. Stack array access words provide a safe alternate
>to doing stack pointer arithmetic in Forth code. Pointer arithmetic
>appears to be the source of a lot of memory safety problems.
At the machine level and the standard Forth level, every array access
performs address arithmetics. Given that standard Forth does not
expose the implementation of the stacks, there is no need to use some
specific implementation for them. One may wonder, though, if using 4
stacks with guard pages around them (i.e., at least 9 pages per task,
set up with 6 system calls) is too expensive for multi-tasking; I
think Gforth currently only does it for the main task.
There are architectures (in particular, the 80286) that provide
hardware support for treating stretches of memory as segments with
bounds checking, and the idea probably was that every array becomes a
segment (not sure about structures; the 80286 supports only 8192
segments, which seems a little low if every struture needs a segment),
but anyway, using segments was too cumbersome, slow and limited, so
they have been let slide by the wayside in the descendents of the
architecture (IA-32, AMD64).
In any case, yes, in Safe Forth there are no addresses at the language
level. You have objects with value-flavoured fields, and arrays with
indexed-fetch and indexed-store words. But in the implementation of
Safe Forth, there will certainly be address arithmetics.
- anton
--
M. Anton Ertl http://www.complang.tuwien.ac.at/anton/home.html
comp.lang.forth FAQs: http://www.complang.tuwien.ac.at/forth/faq/toc.html
New standard: https://forth-standard.org/
EuroForth 2023: https://euro.theforth.net/2023
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