string uppercase and lowercase

; copy uppercase version of string at HL to DE
; output: HL -- Uppercase version
str_upper:
    push hl
    dec de
.loop:
    inc de
    ld c,(hl)
    xor a,a
    cp a,(hl)
    ldi
    jr z,.out
    sbc a,$61
    jr c,.loop
    ld b,a
    ld a,26
    sbc a,b
    jr c,.loop
    ld a,c
    add a,$20
    ld (de),a
    jr .loop
.out:
    pop hl
    ret

; lowercase version of string at HL
; output: HL -- Lowercase version
str_lower:
    push hl
    dec de
.loop:
    inc de
    ld c,(hl)
    xor a,a
    cp a,(hl)
    ldi
    jr z,.out
    sbc a,$61
    jr c,.loop
    ld b,a
    ld a,26
    sbc a,b
    jr c,.loop
    ld a,c
    sbc a,$20
    ld (de),a
    jr .loop
.out:
    pop hl
    ret