Direct Video Memory Access (text) Question

[MXK]

All right, I'm writing a few functions for fast text output in DOS. I'm using the pokeb function in order to write to memory B800:offset, so my line basically looks like this:
pokeb(B800,(y*screen_width)+(x*2),character);
pokeb(B800,(y*screen_width)+(x*2)+1,color);

(Actually I use shifting instead of multiplication but this is easier to understand)

Since the memory is linear the lines are automatically wrapped and the format of the memory is 0:[character], 1:[color], 2:[character], 3:[color],…

That's pretty simple and works fine. My question is as follows, while those two lines seem to be working fine for most computers, on others (my own for example) the start of the video memory is not B800:0 but B800:32. So with that I need to add 32 to the offset in order for the text to appear in the right place, otherwise if x is less then 32 and y is 0 the letters just don't appear. This seems to be machine specific because all of the computers at my school work fine with B800:0 and so do a number of other computers that I've tested this with, but as I've said before, my own computer needs for me to add 32 to the offset. Does anyone know why this is, and if there is a way to detect which setting is needed automatically without me having to create 2 versions of my program that uses this.

[parksie]

Why are you using DOS still?

[MXK]

1. Because it's the best route to learning c++
2. Because that's what we are using at my school
3. Because I love DOS

[jim mcnamara]

Ignore Parksie; he doesn't understand why someone would still have a 386 machine and like it. About 30% of all PC's world wide are 486 or earlier still running DOS and/or Windows 3.x. He means well.

Anyway -
B000 is weird - most video card memory used to start at A000...
as I remember.

Most of the cards running under AMI bios were like this:

MEMORY MAP / ACCESS:
Video Memory Address: A000-BFFF
Video BIOS Address: C000-C7FF
Video Port Address: 3B0-3DF

These may vary - but BIOS knows where video memory starts.
You have to know what BIOS you are looking at, what video card as well.

So - for example, if you have AMI bios:

When you system first boots it displays BIOS information. Record it. Then go to www.ami.com. You can download the memory configuration for your particular BIOS from there. Then peek at the correct BIOS offset to get the start of video memory. This is how DOS finds it.

[MXK]

You're thinking of the 13h mode with A000:0000 as its memory location. In text mode (25x80 or 50x80) the address for the video memory is (or should be) B800:0000, in my case B800:0020.

So is there a way to get this information directly from BIOS? I mean I wouldn't want to do it "if this bios" -> "use this location" type of thing?

[parksie]

Originally posted by jim mcnamara
Ignore Parksie; he doesn't understand why someone would still have a 386 machine and like it. About 30% of all PC's world wide are 486 or earlier still running DOS and/or Windows 3.x. He means well.

Actually my aims are purely selfish. As of yet, I have not seen any good *recent* code come out of attempts to program for DOS, in any kind of maintainable way. And invariably, it's people like you and us that end up having to fix it.

I would happily write code for a 386 if I had one, if it had any kind of decent OS support that was capable of using the CPU to its full potential, and you know as well as I do that 16-bit is limiting even on a 386.

[CornedBee]

It's not the best route for learning C++, but that's not the topic of the thread.

[jim mcnamara]

CB-

What is the 'VBF Countdown 15' thing in your sig?

[jim mcnamara]

Int 10 gives you BIOS information about video.
I don't have a reference here on all the int 10 calls but here are a few samples.

I didn't know you understood interrupts.

----------104F00-----------------------------
INT 10 - VESA SuperVGA BIOS - GET SuperVGA INFORMATION
AX = 4F00h
ESI -> 256-byte buffer for SuperVGA information (see below)
Return: AL = 4Fh function supported
AH = status
00h successful
01h failed
SeeAlso: AX=4F01h

Format of SuperVGA information:
Offset Size Description
00h 4 BYTEs signature ('VESA')
04h WORD VESA version number
06h DWORD pointer to OEM name
0Ah 4 BYTEs capabilities
0Eh DWORD pointer to list of supported VESA and OEM video modes
12h 238 BYTEs reserved
----------104F01-----------------------------
INT 10 - VESA SuperVGA BIOS - GET SuperVGA MODE INFORMATION
AX = 4F01h
CX = SuperVGA video mode
ESI -> 256-byte buffer mode information (see below)
Return: AL = 4Fh function supported
AH = status
00h successful
01h failed
SeeAlso: AX=4F00h,AX=4F02h

Format of mode information:
Offset Size Description
00h WORD mode attributes
bit 0: mode supported
bit 1: optional information available
bit 2: BIOS output supported
bit 3: set if color, clear if monochrome
bit 4: set if graphics mode, clear if text mode
02h BYTE window A attributes
bit 0: exists
bit 1: readable
bit 2: writable
bits 3-7 reserved
03h BYTE window B attributes (as for window A)
04h WORD window granularity in K
06h WORD window size in K
08h WORD start segment of window A
0Ah WORD start segment of window B
0Ch DWORD -> FAR window positioning function (equivalent to AX=4F05h)
10h WORD bytes per scan line
---remainder is optional for VESA modes, needed for OEM modes---
12h WORD width in pixels
14h WORD height in pixels
16h BYTE width of character cell in pixels
17h BYTE height if character cell in pixels
18h BYTE number of memory planes
19h BYTE number of bits per pixel
1Ah BYTE number of banks
1Bh BYTE memory model type
1Ch BYTE size of bank in K
----------104F02-----------------------------
INT 10 - VESA SuperVGA BIOS - SET SuperVGA VIDEO MODE
AX = 4F02h
BX = mode
bit 15 set means don't clear video memory
Return: AL = 4Fh function supported
AH = status
00h successful
01h failed
SeeAlso: AX=4F01h,AX=4F03h

Values for VESA video mode:
00h-FFh OEM video modes (see AH=00h)
100h 640x400x256
101h 640x480x256
102h 800x600x16
103h 800x600x256
104h 1024x768x16
105h 1024x768x256
106h 1280x1024x16
107h 1280x1024x256
108h 80x60 text
109h 132x25 text
10Ah 132x43 text
10Bh 132x50 text
10Ch 132x60 text
----------104F03-----------------------------
INT 10 - VESA SuperVGA BIOS - GET CURRENT VIDEO MODE
AX = 4F03h
Return: AL = 4Fh function supported
AH = status
00h successful
01h failed
BX = video mode (see AX=4F02h)
SeeAlso: AX=4F02h
----------104F04-----------------------------
INT 10 - VESA SuperVGA BIOS - SAVE/RESTORE SuperVGA VIDEO STATE
AX = 4F04h
DL = subfunction
00h get state buffer size
Return: BX = number of 64-byte blocks needed
01h save video states
ES:BX -> buffer
02h restore video states
ES:BX -> buffer
CX = flags for states to save/restore
bit 0: video hardware state
bit 1: video BIOS data state
bit 2: video DAC state
bit 3: SuperVGA state
Return: AL = 4Fh function supported
AH = status
00h successful
01h failed
----------104F05-----------------------------
INT 10 - VESA SuperVGA BIOS - CPU VIDEO MEMORY CONTROL
AX = 4F05h
BH = subfunction
00h select video memory window
DX = window address in video memory (in granularity units)
01h get video memory window
Return: DX = window address in video memory (in gran. units)
BL = window number
00h window A
01h window B
Return: AL = 4Fh function supported
AH = status
00h successful
01h failed
SeeAlso: AX=4F06h,AX=4F07h,AX=7000h/BX=0004h
----------104F06-----------------------------
INT 10 - VESA SuperVGA BIOS 1.1 - GET/SET LOGICAL SCAN LINE LENGTH
AX = 4F06h
BL = function
00h set scan line length
CX = desired width in pixels
01h get scan line length
Return: AL = 4Fh if function supported
AH = status
00h successful
01h failed
BX = bytes per scan line
CX = number of pixels per scan line
DX = maximum number of scan lines
Notes: if the desired width is not achievable, the next larger width will be set
the scan line may be wider than the visible area of the screen
this function is valid in text modes, provided that values are
multiplied by the character cell width/height
SeeAlso: AX=4F01h,AX=4F05h,AX=4F07h
----------104F07BH00-------------------------
INT 10 - VESA SuperVGA BIOS 1.1 - GET/SET DISPLAY START
AX = 4F07h
BH = 00h (reserved)
BL = 00h set display start
CX = leftmost displayed pixel in scan line
DX = first displayed scan line
= 01h get display start
Return: BH = 00h
CX = leftmost displayed pixel in scan line
DX = first displayed scan line
Return: AL = 4Fh if function supported
AH = status
00h successful
01h failed
Note: this function is valid in text modes, provided that values are
multiplied by the character cell width/height
SeeAlso: AX=4F01h,AX=4F05h,AX=4F06h
----------104FFF-----------------------------
INT 10 - VESA SuperVGA BIOS - Everex - TURN VESA ON/OFF
AX = 4FFFh
DL = new state (00h off, 01h on)

[CornedBee]

Originally posted by jim mcnamara
CB-

What is the 'VBF Countdown 15' thing in your sig?

Everytime something really pisses me off here I'll decrement the countdown. When it reaches 0 I'll leave VBF forever and only answer questions on Galahtech anymore.