High score keeper

An arcade machine keeps its best score in a file called best.dat, so the record outlives any one game. Standard input gives a count and then that many scores. For each score, look at the file: if it does not exist yet, the score becomes the first record; if the score is higher than the saved one, it replaces it; otherwise the record stands. At the end, print the record from the file.

With the stdin box as given the program prints:

first score: 120
95 does not beat 120
new best: 180 (was 120)
180 does not beat 180
high score: 180

Talk to the file with system calls (requests the operating system carries out for your program): put the service number in x8result address or system call number (x8): where to write a large returned struct, or the call svc makes; a called routine may overwrite it., the arguments in x0argument and return register (x0-x7): carries the first eight arguments and the return value. to x3argument and return register (x0-x7): carries the first eight arguments and the return value., and run svc 0; the answer comes back in x0argument and return register (x0-x7): carries the first eight arguments and the return value..

• openat (56): x0argument and return register (x0-x7): carries the first eight arguments and the return value. = AT_FDCWD (the current folder), x1argument and return register (x0-x7): carries the first eight arguments and the return value. = the file name, x2argument and return register (x0-x7): carries the first eight arguments and the return value. = the flags, x3argument and return register (x0-x7): carries the first eight arguments and the return value. = the permission mode. The flags are bits, written in octal: 0 opens for reading, and 01101 asks for writing (01), creating the file when it is missing (0100), and emptying it when it is not (01000). A mode of 0644 lets the owner write it and everyone read it. A negative answer means the open failed; otherwise the answer is a file descriptor, the number you pass to the other calls.
• read (63) and write (64): x0argument and return register (x0-x7): carries the first eight arguments and the return value. = the descriptor, x1argument and return register (x0-x7): carries the first eight arguments and the return value. = the address of the memory to read into or write from, x2argument and return register (x0-x7): carries the first eight arguments and the return value. = how many bytes.
• close (57): x0argument and return register (x0-x7): carries the first eight arguments and the return value. = the descriptor.

The file holds the score as 8 raw bytes, exactly as the number sits in memory in saved, so one 8-byte read or write moves the whole thing. Scores can be larger than a 32-bit register holds.

What is checked

• every line for the scores in the stdin box, and for more score lists you cannot see
• the program exits cleanly
• the file work goes through system calls (svcSupervisor call (a system call): svc #0 runs the call whose number is in x8. present)