The theory: To know the value of the character at column X and row Y, you need to look in VRAM at LayoutTableAddress + Y * 80 + X.
In practice: You can find a good tutorial on the Grauw's MAP website (http://map.tni.nl/articles/vdp_tut.php).
You have to initialize the VRAM address to read (and activating the incremental access if you want to read several data), then a simple in a,(0x98) allows to get the VRAM content in the A register.
Not tested but should work:
; Read VRAM at address AHL (17-bit) and return value in A
VRAM_Peek:
rlc h
rla
rlc h
rla
srl h
srl h
di
out (0x99),a
ld a,14+128
out (0x99),a
ld a,l
nop
out (0x99),a
ld a,h
out (0x99),a
nop
ei
in a,(0x98)
ret
Yes, I understand it is an MSX2 example and for me it's ok, but I neet to know how I must use this function, because I don't know how I must put the adress into the ALH. I never used ALH before.
Can you give me an example, please?
Thanks
BTW, you have a BIOS function to do that easily.
Function is named RDVRM and located at address 0x004A.
Just set address in HL, call 0x004A, then you get the return value in A.
If you don't need 100% of the power of an MSX, you have plenty of functions in the BIOS to perform all the operations you might need to create an MSX application.
You have the list here: https://map.grauw.nl/resources/msxbios.php
