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The processor doesn't use DMA, it already has direct access to the memory. DMA is usually for hard disks. It's a buss master system. In other words the processor requests a chunk of data from the hard disk but since the hard disk is slow in comparison to the processor, the processor does not need to wait. It will go on about it's bussiness until the Hard disk signals the processor that data is ready. The processor will tri-state (high impedance like an open circuit) it's address and data busses and go to sleep while the DMA controller takes over all memory addressing functions and dumps the data from the hard disk straight into memory. Now you will have to initialize the DMA controller and there is a small bit of handshake during hard disk exception. You can examine the bios to find them for your particuliar machine(fairly standard code). I dont remember port numbers off hand.
As far as allocating memory in general aside from the DMA, it is all done in the GDT (LDT entries) and local descriptor tables. Set up task state segments as needed for individual tasks.

thinking about the future of my project i came to the question "what about the fpu-stack when task-swithing?". i mean, neither the control word nor the stack itselfs is contained in a task state segment (tss). do i have to dump them all manually and load it manually too? any special function for it?

I think you must do it your self, wasn't there a thread bout win95 not supporting MMX (or was it SSE2), and running two tasks that uses MMX (or SEE2) could compromise the restult/MMx (or SEE2 ) registers. Take a look at how other OSes solves the problem, take a look on linux for instace (too bad most asm is in GAS syntax :rolleyes: luckily I've found a few converters, haven't been able to test 'em yet though, I hope they do what they're supposed to...)

i was a bit busy the last time so i had no time for os-programming - but soon: easter holidays!

@scientica: i found 2 functions for including the fpu in taskswitching:

fsave dest

frstor src

saving the whole fpu-information (including controlwords and statuswords) takes either 94 or 108 bytes (i currently don't know what it depends on)

@all:
new question:
since the gfx-mode almost works i wanted to start coding the mouse. ha! not that easy as i tought. in the ports-listing i found some information using the i/o-port 0x60, but i don't know exactly how i use it.
can anybody point me to some sites where i can find further information?
and: in the listing is written, that all commands are acknowledged by 0xfa; do i have to send it or does the mouse send it?

Port 0x60 used for KeyBoard, not Mouse.;)

To answer the first question. The regs necessary to saving machine status are automatically save during a task switch. It's embedded in the processor so you don't need to worry about it.
The mouse is usually on Comm1 or Comm2 Ports 2f8 and 3f8. It operates just like a modem but if I remember correctly it's only 7 bits no stop,no parity. You have to do some look ahead programming to get smooth operation. I attached a little driver Dat file I wrote sometime ago. You can turn it into a COM file by typing "Debug < mtest.dat". Run it in Dos real mode and it will spit out the codes that the mouse sends to the serial port.

for saving the fpu state i think you should use fxsave and fxrstor because they save the xmm regs too. this always needs 512 bytes of space.

Well, you could have it be optional for each task, or allocate the structure when the task uses floating point operations for the first time.

mmx registers are mapped onto the fpu stack yes, but xmm registers (sse) are seperate.

Well, to send a byte, read port 64 until bit 1 becomes clear. Then write the byte to port 60. To receive a byte, wait until bit 0 of port 64 becomes set, and read the byte from port 60. Write the command to port 64 first.
Use command D1 to set the output port. This has the following format:

76543210

x------- Transfer to/from keyboard

-x------ Keyboard clock

--x----- Mouse buffer full

---x---- Keyboard buffer full

----x--- Mouse clock

-----x-- Transfer to/from mouse

------x- A20 enable

-------x Clear to reset CPU

When data is ready, IRQ1 will be asserted. Read port 64 and check bit 5 to determine the source of the data. A set bit indicates that the data is from the mouse.
Command D4 sends commands to the mouse. After each command, the mouse responds with FA or FE. FE indicates that an error occurred when transferring the last command and that it should be sent again.
The following commands are used to control the mouse:
E6 - Set normal scaling
E7 - Set double scaling
E8 xx - Set resolution (1, 2, 4 or 8 units per mm)
E9 - Read status, resolution and sample rate.
The status has the following format:

76543210

-x------ Polling mode

--x----- Enable

---x---- Scaling

-----x-- Left button pressed

-------x Right button pressed

EA - Send data whenever the mouse is moved or a button is pressed.
EB - Read data. This returns 8 bytes.
Byte 00:

76543210

x------- Vertical overflow

-x------ Horizontal overflow

--x----- Up/down flag

---x---- Left/right flag

------x- Right button pressed

-------x Left button pressed

Byte 2: Movement in horizontal direction
Byte 4: Movement in vertical direction
Byte 6: Scrolling wheel movement
F0 - Do not send data automatically
F2 - Read ID (1 byte, usually 00)
F3 - Set sample rate in Hz.
F4 - Enable
F5 - Disable
F6 - Reset to default settings
FE - Send last result again
FF - Reset mouse