Extracting and replacing packed bit fields

The idea

A packed field occupies a specified range of bits within a register or memory word. To extract bits four through seven, shift a copy right by four and mask its low four bits. Copying first preserves the original word. State the width and signedness of the field; this example treats it as an unsigned four-bit value.

Replacing a field is different from setting selected bits. Clear all bits of the destination field, mask the replacement to its permitted width, shift it into position, and combine with OR. Without clearing first, old one bits can survive a replacement intended to contain zeros. Without constraining the replacement, high bits can spill into neighboring fields. All examples use small in-range shift counts and conceptual register traces.

; Extract bits 4..7 from EAX without changing EAX.
mov eax, 0xAB
mov edx, eax
shr edx, 4
and edx, 0x0F

EAX = 0xAB; EDX = 10.

Shifting moves the upper nibble into the low position, and masking removes unrelated bits.