=================================================
c-code:
   *MAPCTL = 0; // memory mapping for boot state
    ptr = (u8*) 0xfd00; // timers and audio fd00
    count = 0x40;//40
    while (count--)
    {
        *ptr++ = 0;
    }
=================================================
What it does:
set mapping to 0 (all ROM, not RAM)
then reset all registers at $fd00 (64).
=================================================

    stz     $FFF9 ;fastes way! good

    ldx     #$FD  ;this sets helper variables
    stz     L0050
    stx     L0050+1
    lda     #$40
    sta     L0051
    stz     L0051+1


    bra     L00D9         ;weirdo: branch away for preparations first

L00D7:
    lda     L0050
    ldx     L0050+1
    sta     regsave
    stx     regsave+1  ;update regsave with helper variables
    ina
    bne     L00DC
    inx
L00DC:
    sta     L0050       ;update helper variables
    stx     L0050+1
    lda     #$00
    sta     (regsave)  ;FINALLY write 0 to the register

L00D9:
    lda     L0051   ;read helper variabels and store them in regsave
    ldx     L0051+1
    sta     regsave
    stx     regsave+1
    jsr     decax1     ;prepare counter
    sta     L0051
    stx     L0051+1
    lda     regsave    ;...whatever
    ora     regsave+1
    bne     L00D7      ;branch back as part of the loop now!
=================================================
Assembler:
    stz     $FFF9 ;identical to compiled part
    ldx #$40
    lda #0
loop
    sta $fd00,x ;you can't do STZ blabla,x
    dex
    bpl loop