J3/98-111

Date:     29 Jan 1998
To:       J3
From:     Dick Hendrickson
Subject:  4d:  control of I/O rounding, specs and syntax

This is the Specs for item 4d, Control of I/O Rounding, for Interval
Arithmetic Enabling Technologies.  Assuming there is general agreement, I
think this is detailed enough to also be the syntax document.

The basic idea is to add new rounding control facilities modeled after BN
and BZ.  They will control the way a processor must round real and complex
values for F, E, EN, ES, D, and G formatting, both on input and on output.

There are 4 modes provided:
DRU          Round up
DRD          Round down
DRZ          Round to zero
DRN          Round to nearest

The intent is to mimic the IEEE rounding modes, but not actually require
IEEE arithmetic.  We'll need explicit words to say that, for example, DRU
must produce an internal value that is greater than or equal to the input
character string or a character value that is greater than or equal to the
internal value.  We don't want to force any particular accuracy
requirements, just the bounds.

Directed rounding only applies to files connected for formatted I/O.

There are 4 areas that need to be changed.

OPEN
Add a new connect-spec (R905)
ROUND = scalar-default-char-expression
The expression shall be one of "DRU", "DRD", "DRZ", or "DRN"

This specifies the initial rounding for a file connection.

If no ROUND= is present, the rounding mode is processor dependent.  It's
not clear to me what this means.  Do we want the processor to pick one of
the above methods and use it all of the time?  Or can it use a proprietary
method that isn't exactly any of the above?

INQUIRE
Add a new  inquire-spec (R924)
ROUND = scalar-default-char-variable
Lets you find out the rounding mode in effect.
Returns one of the above strings, or "UNDEFINED" if not specified on the OPEN

READ/WRITE
Add round-edit-desc to the format control-edit-desc (R1010) with the four
above possible values.  It wraps around when a closing parenthesis is
reached just like other control edit descriptors.

Like BN, once you start using a rounding mode in a format there is no way
to return to the default.

DERIVED TYPE I/O
The current rounding mode is one more item (like BN, SS...) that needs to
be saved when a user derived type I/O routine is entered and restored when
it exits.  It's unclear to me whether the rounding mode is set to the
processor default value or to the OPENed value when the user routine is
called.  We should do it the way the similar descriptors are handled.

Potential straw votes.

1)  Should we provide all 4 (only the first 2 are really needed for
interval arithmetic)

2)  Should we provide more?

3)  Should we define the processor default?

4)  Should we spell "DRU" "differently" (if so, how do we spell the others?)

5)  Should we add "none of the above" as a result for INQUIRE when the
processor uses a proprietary method or should we require the processor to
use one of these methods?

6)  On OPEN should ROUND be ignored for unformatted I/O or should it be an
error?

7)  Should INQUIRE return UNDEFINED if the OPEN didn't specify a method or
may it return one of the above if that's what it actually uses?

8)  Should INQUIRE return UNDEFINED if the file wasn't opened for formatted
I/O?

9)  What should the mode be when a user derived type I/O routine is entered?