sys-file-reader: Accept measurement value of zero without complaint.

[pspp] / doc / dev / system-file-format.texi

diff --git a/doc/dev/system-file-format.texi b/doc/dev/system-file-format.texi
index 7b0eff9f79823f0bb623e10d2f1da8c22922eff7..af74fd2f600c6e95f0380afa23465605365263b8 100644 (file)
--- a/doc/dev/system-file-format.texi
+++ b/doc/dev/system-file-format.texi
@@ -33,12 +33,28 @@ floating-point numbers, and translates as needed.  However, only IEEE
 has actually been observed in system files, and it is likely that
 other formats are obsolete or were never used.
 
-The PSPP system-missing value is represented by the largest possible
-negative number in the floating point format (@code{-DBL_MAX}).  Two
-other values are important for use as missing values: @code{HIGHEST},
-represented by the largest possible positive number (@code{DBL_MAX}),
-and @code{LOWEST}, represented by the second-largest negative number
-(in IEEE 754 format, @code{0xffeffffffffffffe}).
+System files use a few floating point values for special purposes:
+
+@table @asis
+@item SYSMIS
+The system-missing value is represented by the largest possible
+negative number in the floating point format (@code{-DBL_MAX}).
+
+@item HIGHEST
+HIGHEST is used as the high end of a missing value range with an
+unbounded maximum.  It is represented by the largest possible positive
+number (@code{DBL_MAX}).
+
+@item LOWEST
+LOWEST is used as the low end of a missing value range with an
+unbounded minimum.  It was originally represented by the
+second-largest negative number (in IEEE 754 format,
+@code{0xffeffffffffffffe}).  System files written by SPSS 21 and later
+instead use the largest negative number (@code{-DBL_MAX}), the same
+value as SYSMIS.  This does not lead to ambiguity because LOWEST
+appears in system files only in missing value ranges, which never
+contain SYSMIS.
+@end table
 
 System files are divided into records, each of which begins with a
 4-byte record type, usually regarded as an @code{int32}.
@@ -115,7 +131,9 @@ char                padding[3];
 
 @table @code
 @item char rec_type[4];
-Record type code, set to @samp{$FL2}.
+Record type code, set to @samp{$FL2}, that is, either @code{24 46 4c
+32} if the file uses an ASCII-based character encoding, or @code{5b c6
+d3 f2} if the file uses an EBCDIC-based character encoding.
 
 @item char prod_name[60];
 Product identification string.  This always begins with the characters
@@ -559,6 +577,9 @@ Machine endianness.  1 indicates big-endian, 2 indicates little-endian.
 been actually observed in system files:
 
 @table @asis
+@item 1
+EBCDIC.
+
 @item 2
 7-bit ASCII.
 
@@ -579,9 +600,6 @@ UTF-8.
 The following additional values are known to be defined:
 
 @table @asis
-@item 1
-EBCDIC.
-
 @item 3
 8-bit ``ASCII''.
 
@@ -591,9 +609,10 @@ DEC Kanji.
 
 Other Windows code page numbers are known to be generally valid.
 
-Old versions of SPSS always wrote value 2 in this field, regardless of
-the encoding in use.  Newer versions also write the character encoding
-as a string (see @ref{Character Encoding Record}).
+Old versions of SPSS for Unix and Windows always wrote value 2 in this
+field, regardless of the encoding in use.  Newer versions also write
+the character encoding as a string (see @ref{Character Encoding
+Record}).
 @end table
 
 @node Machine Floating-Point Info Record
@@ -812,8 +831,8 @@ Ordinal Scale
 Continuous Scale
 @end table
 
-SPSS 14 sometimes writes a @code{measure} of 0 for string variables.
-PSPP interprets this as nominal scale.
+SPSS sometimes writes a @code{measure} of 0.  PSPP interprets this as
+nominal scale.
 
 @item int32 width;
 The width of the display column for the variable in characters.
@@ -1234,7 +1253,9 @@ Record type.  Always set to 7.
 @item int32 subtype;
 Record subtype.  May take any value.  According to Aapi
 H@"am@"al@"ainen, value 5 indicates a set of grouped variables and 6
-indicates date info (probably related to USE).
+indicates date info (probably related to USE).  Subtype 24 appears to
+contain XML that describes how data in the file should be displayed
+on-screen.
 
 @item int32 size;
 Size of each piece of data in the data part.  Should have the value 1,