Product specifications

BASIC sets up the VRAM tables as shown in Appendix A. The data bytes which are

used, and the routine to perform this set-up are listed in the example below.

The routine is called VRGINI (VDP Registers Initialisation Routine). It needs no

which has 18 elements, and the routine VOUTRG, which must be present.

;VRGINI-INITIALISE VDP WRITE-ONLY REGISTERS DEPENDENT CONTENTS OF

TABLE VRGTAB

; NO REGISTER SET-UP REQUIRED ON ENTRY

; NO REGISTERS AFFECTED ON EXIT

;

VRGINI: PUSH AF ;Save Ace and flags

PUSH DE ;Save DE register pair

PUSH HL ;Save HL register pair

LD B, 18 ;Set loop counter = 18

LD HL,VRGTAB ;Set HL to point to start address of table

;VRGTAB

VRG1: LD E,(HL) ;Load data byte into E

INC HL ;Move onto next byte in table VRGTAB

LD D, (HL) ;Load control byte into D

INC HL ; Move onto next byte in table VRGTAB

CALL VOUTRG ;Output data byte held in to register number

;held in D

DJNZ VRG1 ;Decrement loop counter and If loop counter

;<> 0 then goto VRG1 else drop through to

;VRG2

VRG2: POP HL ;Get old HL register pair

POP DE ;Get old DE register pair

POP AF ;Get old Acc and flags

RET ;Return to calling routine

;

VRGTAB: DB 0,0,0,0,0,0,0,0,0

DB 0,0,0,0,0,0,0,0,0

;

The contents of the eight VDP write-only registers are shown in the table below.

Following this table is a description of each register and what each of the control bits

actually do.

Registers 0 and 1 contain flags to enable or disable various VDP features and modes.

Registers 2 through to 6 contain values which are the start addresses for the various

sub-blocks of VRAM (for example where the screen is located).