Bit fields: ext and ins

The day before yesterday you got fields out with srl and and. Two instructions do that in one line – and the opposite way too.

ext: pulling a field out#

ext r? a start length takes the field starting at bit start with length bits out of a and puts it right-aligned into the register:

ext r0 %1101_0110 4 4   # bits 4–7: %1101 = 13
ext r1 %1101_0110 1 2   # bits 1–2: %11 = 3

That equals srl plus and with a mask of length ones.

ins: putting a field in#

ins r? value start length writes the lower length bits of value into the register from bit start on and leaves all other bits alone. The register is target and starting value:

move r0 $FF          # %1111_1111
ins r0 0 2 3         # bits 2–4 to 0: 227
ins r0 5 2 3         # bits 2–4 to %101: 247

If value is too big for the field, ins cuts it off: only the lower length bits go in.

At most 53 bits#

A field must not reach beyond bit 52: start + length at most 53, length at least 1. Otherwise the chip stops with an error (PayloadOverflow, ShiftOverflow or ShiftUnderflow). That is the 53-bit limit of the decimal number from Monday.

Packing values#

With fields you pack several small values into one number. An example for a pump station:

13 bits for three values: one register or one stack slot instead of three. Next week you use that to send values between chips.