Assemble it yourself

Be the assembler for one instruction: add xD, xN, imm, a 64-bit add with an immediate. Standard input gives D, N and the immediate; build the 32-bit word the assembler would produce and print it. With the stdin box as given the program prints:

probe check: ok
add x3, x4, 100 = 0x91019083

The add (immediate) format, from bit 31 down:

Bits 31 to 22 never change here, so the word starts as 0x91000000 (one movzMove wide zero-extended; writes imm << hw*16, zeros the rest. with lsl 16 can load it). Shift each field up to its place with lslLogical shift left (Rd = Rn << amount). and combine them with orrBitwise OR; register or bitmask immediate.. Write that as a subroutine, encode, because it runs twice:

1. The starter holds a real add x1, x2, 42 at the label probe. It never runs, but its four bytes can be loaded like any other word. Encode Rd = 1, Rn = 2 and 42 yourself and print probe check: ok when your word matches the assembler's, probe check: mismatch when it does not.
2. Then encode the input and print the instruction and its word.

A register number is 0 to 31, and in this format 31 means spstack pointer (sp): points to the top of the stack; keep it a multiple of 16. (print_reg already prints it that way). The immediate field has 12 bits, so it holds 0 to 4095. Anything outside those ranges, negatives included, prints cannot encode that and returns 1.

What is checked

• both lines for the input in the stdin box, and for more inputs you cannot see
• the exit status (0, or 1 when the input cannot be encoded)
• the word is built with lslLogical shift left (Rd = Rn << amount). and orrBitwise OR; register or bitmask immediate.