Product specifications

After the address set up has been made, data bytes can either be input or output
along port 1, and because the VRAM is managed by an auto-incrementing register,
sequential transfers of data bytes can be performed without having to re-set the
address pointer on the VDP chip.
If you wish to perform alternate input and output of data bytes to VRAM you must re-
set the addressing mode as appropriate.
Address set ups and data transfers require a certain minimum a of time between
sequential processes. This is 11 micro-seconds between sequential address set-ups,
and 8 micro-seconds between sequential data transfers.
If you are using BASIC there will never be any problem, but if you are using machine
code this is a point to be aware of.
Listed in the next two sections are routines which will perform the job of "PEEKING"
and "POKEING" to the video screen.
Section 1.4 describes the BASIC routines. Section 1.5 and 1.6 describe the assembler
code.
1.4
Both of these sections assume that you are using 'TEXT MODE' ie VS 5, which is the
normal default BASIC VRAM screen set-up.
For BASIC VRAM memory map see Appendix A..
100 REM THESE ROUTINES USE TWO VARIABLES
VADDRESS - USED TO SET UP VRAM ADDRESS
VDATA - USED TO RECEIVE OR SEND SCREEN DATA
110 LET VADDRESS=7*1024: LET VDATA=42
(Set up VRAM address pointer to 7K and data byte to numerical value for ‘*’)
120 GOSUB 1000
(POKE byte onto the screen)
130 LET ADDRESS=7*1024
(Set up VRAM address pointer to 7K)
140 GOSUB 2000 : PRINT VDATA
(PEEK byte on screen in location pointed by VADDRESS, set up in line 130, and print
result)