from http://www.usm.uni-muenchen.de/people/puls/f77to90/cray.html:

REAL                        half  Default,single          double
  KIND number =                4               8              16
       digits =               24              47              96
  maxexponent =             8189            8189            8190
  minexponent =            -8188           -8188           -8189
    precision =                6              13              28
        radix =                2               2               2
        range =             2465            2465            2465
      epsilon =   0.11920929E-06   0.14210855-13  0.25243549E-28
         tiny =  0.73344155-2465 0.73344155-2465 0.73344155-2465
         huge =  0.13634352+2466 0.13634352+2466 0.13634352+2466

This seems to indicate a 32-bit float with the same 15-bit exponent, and a 16-bit coefficient? Others seem to indicate a 24-bit coefficient, which would make sense being a 'half' precision, but that doesn't allow for much of an exponent in 32-bits. Puzzling...

There's a ruby package that implement the Cray (full) float format, with test code as well (http://rubyforge.org/frs/?group_id=4684). Here's how they define it:

# Cray-1
Flt.define :CRAY, BinaryFormat,
  :fields=>[:significand,48,:exponent,15,:sign,1],
  :bias=>16384, :bias_mode=>:fractional_significand,
  :hidden_bit=>false,
  :min_encoded_exp=>8192, :max_encoded_exp=>24575, :zero_encoded_exp=>0,
  :endianness=>:big_endian,
  :gradual_underflow=>false, :infinity=>false, :nan=>false

And here's some test code:
CRAY:
  parameters:
  - total_bits: 64
  - radix: 2
  - significand_digits: 48
  - radix_min_exp: -8193
  - radix_max_exp: 8190
  - decimal_digits_stored: 14
  - decimal_digits_necessary: 16
  - decimal_min_exp: -2466
  - decimal_max_exp: 2466
  values:
  - Rational(1, 3): 3F FF AA AA AA AA AA AB
  - Rational(1, 10): 3F FD CC CC CC CC CC CD
  - Rational(2, 3): 40 00 AA AA AA AA AA AB
  - Rational(1, 1024): 3F F7 80 00 00 00 00 00
  - Rational(1, 1000): 3F F7 83 12 6E 97 8D 50
  - Rational(1024, 1): 40 0B 80 00 00 00 00 00
  - Rational(1024, 1): 40 0B 80 00 00 00 00 00
  special:
  - min_value: 20 00 80 00 00 00 00 00
  - min_normalized_value: 20 00 80 00 00 00 00 00
  - max_value: 5F FF FF FF FF FF FF FF
  - epsilon: 3F D2 80 00 00 00 00 00
  - strict_epsilon: 3F D2 80 00 00 00 00 00
  numerals:
  - "+0": 00 00 00 00 00 00 00 00
  - "-0": 80 00 00 00 00 00 00 00
  - "+1": 40 01 80 00 00 00 00 00
  - "-1": C0 01 80 00 00 00 00 00
  - "+0.1": 3F FD CC CC CC CC CC CD
  - "-0.1": BF FD CC CC CC CC CC CD
  - "0.5": 40 00 80 00 00 00 00 00
  - "-0.5": C0 00 80 00 00 00 00 00
  - "29.2": 40 05 E9 99 99 99 99 9A
  - "-29.2": C0 05 E9 99 99 99 99 9A
  - "0.03125": 3F FC 80 00 00 00 00 00
  - "-0.03125": BF FC 80 00 00 00 00 00
  - "-0.3125": BF FF A0 00 00 00 00 00
  - 1.234E2: 40 07 F6 CC CC CC CC CD
  - "-1.234E-6": BF ED A5 9F EA CA 16 6D
  - "65536.0": 40 11 80 00 00 00 00 00
  - "-65536.0": C0 11 80 00 00 00 00 00
  - "-7.50": C0 03 F0 00 00 00 00 00
  - "4.584009668887118E-2467": 20 00 80 00 00 00 00 00
  - 5.45374067809706E2465: 5F FF FF FF FF FF FF FF
  - "7.105427357601002E-15": 3F D2 80 00 00 00 00 00
  - "7.105427357601002E-15": 3F D2 80 00 00 00 00 00
  base: :bytes


From the Cray reference manual:

'The 18 low-order bits of the half-precision results are returned as zeros with a round applied to the low-order bit of the 30-bit result'

This indicates a 30-bit coefficient, and a 15-bit exponent. It also means that half-precision numbers are represented the same way as normal floats (i.e. in 64-bits) only results are computed faster.