Basic Structures

Strings

Offsets

At many places, other locations in the file are referenced. These locations are given as Offsets: the number of bytes from the start of the file. Offsets are always encoded in Longs.

Lengths

Lengths are encoded within longs. The unit is a twip, or 1/5A0 inch (1/1440 decimal). This means 237 (567 decimal) units correspond to 1 cm.

Sizes

Colors

Colors are encoded within three bytes. Each byte ranges from 00 (black) to FF (white). This is clearly intended to be a RGB encoding, but the Psion 5 only has a greytone display. All three bytes should be equal to express a greytone, and in practice, only values 00 00 00, 55 55 55, AA AA AA and FF FF FF are seen.

Signed Integers

Signed integers are encoded in longs. The most significant bit is used as sign (0 for positive, 1 for negative). So +1 is encoded as 01 00 00 00, and -1 as 01 00 00 80.

Floating Point Numbers

Floating point numbers are encoded in 8 bytes. The most significant bit is used as sign (0 for positive, 1 for negative). The next B (11 decimal) bits are used as a two-complement exponent, and the remaining 34 (52 decimal) bits are used as the mantissa.

The complete number can be found through this C formula (>> means shift right, ** means to the power of, Float is the 8 byte unsigned integer representation):

(Float & 0x8000000000000000 ? -1 : 1) *

(1 + (Float & 0x000FFFFFFFFFFFFF) / 0x0010000000000000) *

(2 ** (((Float & 0x7FF0000000000000) >> 52) - 0x3FF)

Some example representations:

Number (decimal) Sign bit Exponent Mantissa Complete

1.0 0 3FF 0000000000000 00 00 00 00 00 00 F0 3F

2.0 0 400 0000000000000 00 00 00 00 00 00 00 40

3.0 0 400 8000000000000 00 00 00 00 00 00 08 40

3.5 0 400 C000000000000 00 00 00 00 00 00 0C 40

-3.5 1 400 C000000000000 00 00 00 00 00 00 0C C0

0.5 0 3FE 0000000000000 00 00 00 00 00 00 E0 3F

0.0 0 000 0000000000000 00 00 00 00 00 00 00 00