Comments added.
Tested on x86_64-pc-linux-gnu, committed on trunk
2012-11-06 Tristan Gingold <ging...@adacore.com>
* exp_vfpt.adb: Document VAX float point layout.
Index: exp_vfpt.adb
===================================================================
--- exp_vfpt.adb (revision 193215)
+++ exp_vfpt.adb (working copy)
@@ -6,7 +6,7 @@
-- --
-- B o d y --
-- --
--- Copyright (C) 1997-2010, Free Software Foundation, Inc. --
+-- Copyright (C) 1997-2012, Free Software Foundation, Inc. --
-- --
-- GNAT is free software; you can redistribute it and/or modify it under --
-- terms of the GNU General Public License as published by the Free Soft- --
@@ -37,6 +37,78 @@
package body Exp_VFpt is
+ -- Vax floating point format (from Vax Architecture Reference Manual
+ -- version 6):
+ --
+ -- Float F:
+ -- --------
+ --
+ -- 1 1
+ -- 5 4 7 6 0
+ -- +-+---------------+--------------+
+ -- |S| exp | fraction | A
+ -- +-+---------------+--------------+
+ -- | fraction | A + 2
+ -- +--------------------------------+
+ --
+ -- bit 15 is the sign bit,
+ -- bits 14:7 is the excess 128 binary exponent,
+ -- bits 6:0 and 31:16 the normalized 24-bit fraction with the redundant
+ -- most significant fraction bit not represented.
+ --
+ -- An exponent value of 0 together with a sign bit of 0, is taken to
+ -- indicate that the datum has a value of 0. Exponent values of 1 through
+ -- 255 indicate true binary exponents of -127 to +127. An exponent value
+ -- of 0, together with a sign bit of 1, is taken as reserved.
+ --
+ -- Note that fraction bits are not continuous in memory, VAX is little
+ -- endian (LSB first).
+ --
+ -- Float D:
+ -- --------
+ --
+ -- 1 1
+ -- 5 4 7 6 0
+ -- +-+---------------+--------------+
+ -- |S| exp | fraction | A
+ -- +-+---------------+--------------+
+ -- | fraction | A + 2
+ -- +--------------------------------+
+ -- | fraction | A + 4
+ -- +--------------------------------+
+ -- | fraction | A + 6
+ -- +--------------------------------+
+ --
+ -- Like Float F but with 55 bits for the fraction.
+ --
+ -- Float G:
+ -- --------
+ --
+ -- 1 1
+ -- 5 4 4 3 0
+ -- +-+---------------------+--------+
+ -- |S| exp | fract | A
+ -- +-+---------------------+--------+
+ -- | fraction | A + 2
+ -- +--------------------------------+
+ -- | fraction | A + 4
+ -- +--------------------------------+
+ -- | fraction | A + 6
+ -- +--------------------------------+
+ --
+ -- Exponent values of 1 through 2047 indicate trye binary exponents of
+ -- -1023 to +1023.
+ --
+ -- Main differences compared to IEEE 754:
+ --
+ -- * No denormalized numbers
+ -- * No infinity
+ -- * No NaN
+ -- * No -0.0
+ -- * Reserved values (exp = 0, sign = 1)
+ -- * Vax mantissa represent values [0.5, 1)
+ -- * Bias is shifted by 1 (for single float: 128 on Vax, 127 on IEEE)
+
VAXFF_Digits : constant := 6;
VAXDF_Digits : constant := 9;
VAXGF_Digits : constant := 15;
