z00m's speccy knowledge base

downde  inc d
        ld a,d
        and 7
        ret nz
        ld a,e
        add a,#20
        ld e,a
        ret c
        ld a,d
        sub 8
        ld d,a
        ret

Slower alternative:

downde    inc  d                 
          ld   a,d               
          and  7                 
          ret  nz                
          ld   a,e               
          add  a,32              
          ld   e,a               
          ld   a,d               
          jr   c,downde2         
          sub  8                 
          ld   d,a               
downde2   cp   88                
          ret  c                 
          ld   d,64              
          ret