Several values in one number

A logic memory holds exactly one number. If you want to pass temperature, pressure and oxygen of a room on to another chip, you need three memories – or you pack everything into one number. With ins and ext from bit week that is no witchcraft.

Planning fields#

First the question: what range does each value have, and how many bits does it need?

n bits hold numbers from 0 to 2^n − 1. Together that is 22 bits – far below the limit of 53.

Packing with ins#

Each value is rounded, limited to its range and then inserted. clamp makes sure an outlier does not spill into the neighbouring field:

sub value value CELSIUSOFFSET
round value value
clamp value value 0 127     # fits into 7 bits
ins packed value 0 7

Unpacking with ext#

The receiver pulls the fields out again with the same numbers:

ext temp packed 0 7
ext press packed 7 8
ext o2 packed 15 7

Write the field table as a comment into both chips. If a field changes on one side only, the receiver reads nonsense without any error.

Shifting negative values#

A field knows no negative numbers. On Mars it is well below 0 °C outside at night. So shift beforehand: °C + 50 turns −50 to 77 °C into the range 0 to 127. When unpacking you subtract the 50 again.

Saving space in the stack#

The stack has 512 slots. A history with three values per reading lasts for 170 readings unpacked, for 512 packed – three times as long:

push packed                # one slot for three values