i may not be qualified to answer all those questions for you, but i'll
try my best. i was dealing with FP exceptions last week and
consequently may have some of the answers:
1) Is there a Plan 9 policy concerning the "notes" (exceptions) on
arithmetic operations, both integer and float?
i don't think there's anything about integers, but float exception
behaviour can be controlled via the getfcr/setfcr routines described
in getfcr(2). that man page will also tell you how to figure out
whether a particular machine supports those.
in my particular case the code I was porting defined NaN as a constant
and threw an exception whenever it ran. one particular quirk that i
observed was that an exception was thrown regardless of whether I
defined a silent or a non-silent NaN... my "solution" was simply to
turn off FPINVAL via setfcr(). if you're porting via ape, as i was
doing, the best solution is to get the little bits of assembly that
constitute the *fcr routines, compile them with 8a and link them
against the
2) Concerning float, if the FPU is a (from the ISA point of
view) separate entity, are registers put in a known state on process
entry?
yes. here are the floating point registers of a brand spanking new process:
parr% acid /bin/links
/bin/links:386 plan 9 executable
/sys/lib/acid/port
/sys/lib/acid/386
acid: new()
18031: system call _main SUBL $0xc,SP
18031: breakpoint main+0x3 MOVL $_IO_stream+0x40(SB),AX
acid: fpr()
F0 0
F1 0
F2 0
F3 0
F4 0
F5 0
F6 0
F7 0
control 0x0000
status 0x0000
tag 0x0000
ip offset 0x00000000
cs selector 0x0000
opcode 0x81f3
data operand offset 0x0000
operand selector 0x0000
3) Is there some high level arithmetic homogeneous behavior between
machines---a IEEE754 behavior even on machines lacking a compliant FPU,
or lacking a FPU?
i believe the answer is No.
4) Is there a way, via C, to handle extended precision on IEEE754, or is
assembly required (even if the assembler does not provide directly the
symbolic opcodes, embedding directly the values is possible)?
not sure if that's what you're asking exactly, but from getfcr(2):
Precision is
controlled by installing in fcr, under mask FPPMASK, one of
the values FPPEXT (extended precision), FPPSGL (single pre-
cision), and FPPDBL (double precision).
hope that helps.
