There are really few systems where you can save a part of a byte! And if you need to output a byte anyway, it doesn't matter which byte it is. So you can indulg

There are really few systems where you can save a part of a byte! And if you need to output a byte anyway, it doesn't matter which byte it is. So you can indulge and use "?", "!", "*", or even "&" to signify various types of error conditions. (On certain architectures, you could use 1-byte soft-interrupt opcodes to call the most used subroutine, but 8080 lacked it IIRC; on 6502 you could theoretically use BRK for that. But likely you had other uses for it than printing error diagnostics.)