Now I learn about to prepare all routines to support console output mode redirect to VDP in textmode. Until now can view all text printed out on serial port and VDP display, solved bugs into my first text scroll up routine. 

Try with simple BASIC program scrollup correct effect:

Change into RST 08 service routine to putchar to VDP and send to Serial:

          .ORG     0008H
RST08     DI
          CALL     VDP_PUTCHAR
          EI
          JP       TXA

Create main service routine dispatcher RST 20, this is a firmware main service routine that dispatch any firmware function, like: VDP or PSG control. 

For example, to change screen mode:

LD      A, 0      ; Mode 0 (TEXTMODE)
LD      B, 3      ; Call service routine number 1 (VDP_SET_MODE)
RST     $20    

 Or change screen color:

 LD      A, $F5   ; White foreground and light blue background
 LD      B, 0     ; Call service routine number 0 (VDP_SETCOLOR)
 RST     $20