Need support for splitting an integer.

[j3pic]

Consider the EDID format used by HDMI monitors: https://en.wikipedia.org/wiki/Extended_Display_Identification_Data#EDID_1.4_data_format

Part of this spec calls for an integer whose lowest 8 bits are followed by the lowest 8 bits of an unrelated integer, followed by the highest 4 bits of the first integer, then the highest 4 bits of the second integer.

Something like this:

(defbinary troll-format (:byte-order :big-endian)
    (number-of-frobs 0 :type (unsigned-byte 8))
    (unrelated-cruft 0 :type (unsigned-byte 8))
    (rest-of-number-of-frobs 0 :type (unsigned-byte 4)) ;; u mad bro?
    (rest-of-unrelated-cruft 0 :type (unsigned-byte 4)))

I've seen other formats that do things like this (but never an explanation of why someone would do
this), so perhaps there should be some way to specify that REST-OF-NUMBER-OF-FROBS is part of NUMBER-OF-FROBS (and which part it is in that case, since someone could specify a format in which the chunks are laid out in middle-endian order).

[j3pic]

One approach that seems reasonably elegant is to have :before and :after options that could be used to sequence fragments relative to other named fragments of the same integer.

So the troll-format above could be written like this:

(defbinary troll-format (:byte-order :big-endian)
    (number-of-frobs 0 :type (unsigned-byte 8))
    (unrelated-cruft 0 :type (unsigned-byte 8))
    (rest-of-number-of-frobs 0 :type (unsigned-byte 4 :after number-of-frobs)) 
    (rest-of-unrelated-cruft 0 :type (unsigned-byte 4 :after unrelated-cruft))
    (middle-of-number-of-frobs 0 :type (unsigned-byte 8 :before rest-of-number-of-frobs))

The macro would detect the presence of :before and :after fragments and
generate code that shifts and masks each fragment into the right place upon reading/writing.

[j3pic]

I've found another real-world data format that splits integers across non-adjacent bytes. The x86 architecture's global descriptor table is described on page 38 here: https://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf

[j3pic]

Perhaps a more readable syntax would be to give the fragments numbers instead of :before and :after tags:

(defbinary troll-format (:byte-order :big-endian)
    (number-of-frobs 0 :type (unsigned-byte 8 :fragment 0))
    (unrelated-cruft 0 :type (unsigned-byte 8 :fragment 1))
    (rest-of-number-of-frobs 0 :type (unsigned-byte 4 :fragment-of (number-of-frobs 2)
    (rest-of-unrelated-cruft 0 :type (unsigned-byte 4 :fragment-of (unrelated-cruft 0)))
    (middle-of-number-of-frobs 0 :type (unsigned-byte 8 :fragment-of (number-of-frobs 1)))

The primary variable would have a :fragment in its type indicating which position it's in. The secondary variables
would use :fragment-of to specify both the primary variable to mask into, and the position relative to other variables.

This scheme would be inadequate if there was an array that interleaved fragments of its elements.