Clear vertical line counter of GA via bit 4 of Reg 2

vlc-reset.asm

        org     &8000
        run     &8000
        nolist

        jp      start

vsync:
        ld      b, &f5
        in      a, (c)
        rra
        jr      nc, vsync
        ret

start:
        di

; Put EI RET at &38
        ld      a, &fb
        ld      (&38), a
        ld      a, &c9
        ld      (&39), a

; Disable Lower and Upper ROM
        ld      bc, &7f00 or &80 or &08 or &04 or &1
        out     (c), c

        ei

loop:
        call    vsync
        halt
        halt
        halt
        ld      bc, &7f00 or &10
        out     (c), c
        ld      bc, &7f00 or &40 or &00
        out     (c), c

repeat  64*15
        nop
rend

; Set bit 4 of Register 2 in GA
        ld      bc, &7f00 or &90 or &08 or &04 or &1
        out     (c), c

        halt

        ld      bc, &7f00 or &10
        out     (c), c
        ld      bc, &7f00 or &40 or &04
        out     (c), c

        jp      loop