Every opcode packs its operands into its sixteen bits, and decoding means slicing those bits into named fields. Today you write the pure function that splits an opcode into the x, y, n, nn, and nnn fields every instruction reads.
Decode a 16-bit opcode into its standard fields x, y, n, nn (kk), and nnn.
CHIP-8’s instruction encoding is wonderfully regular. Read an opcode as four nibbles (4-bit groups) and the operands always live in the same places. The top nibble usually selects the instruction; the remaining nibbles are the arguments, and different instructions read them at different widths. The five standard fields cover every case: x (second nibble) and y (third nibble) name registers, n (last nibble) is a 4-bit value, nn (last byte, also written kk) is an 8-bit value, and nnn (last three nibbles) is a 12-bit address.
Decoding is pure bit-shifting and masking with no state, which makes it a perfect standalone function to test in isolation. Take 0xD123: it is the draw instruction, x is 1, y is 2, and n is 3 (draw a 3-row sprite). The same sixteen bits, read as nnn, give the address 0x123. Getting these masks exactly right now means every opcode you implement from here just names the field it wants.
// opcode bits, high to low: [ high | x | y | n ] (four nibbles)func decode(op uint16) (x, y, n byte, nn byte, nnn uint16) {x = byte(op >> 8 & 0x0F)y = byte(op >> 4 & 0x0F)n = byte(op & 0x0F)nn = byte(op & 0x00FF)nnn = op & 0x0FFFreturn}