For example, the loop here doesn't work:
[code] ld bc,&f40e ;; PSG register 14 (keyboard) out (c),c ld bc,&f6c0 out (c),c ld bc,&f600 out (c),c ld bc,&f792 ;; PPI port A input out (c),c update_loop: ld b,&f4 in a,(c) jp update_loop[/code]
whereas the following does work:
[code] ld bc,&f40e ;; PSG register 14 (keyboard) out (c),c ld bc,&f6c0 out (c),c ld bc,&f600 out (c),c ld bc,&f792 ;; PPI port A input out (c),c update_loop: ld bc,&f649 out (c),c ld b,&f4 in a,(c) ld bc,&f640 out (c),c jp update_loop[/code]