INTRO TO DMA by Draeden of VLA



DMA means Direct Memory Access. You probably already know where and
why you use it, so I'll skip right down to the dirty stuff. This all
should speak for it's self, so... Enjoy.

Draeden /VLA



To do a DMA transfer, you need to know a few things:

1) Address of the memory to access

2) Length of data to read/write

This can all be put into a structure:

STRUC DMAInfo
Page db ?
Offset dw ?
Length dw ?
ENDS

Page is the highest 4 bits of the absolute 20 bit address of the memory
location. Note that DMA transfers CANNOT cross 64k page boundries.

The Length is actually LENGTH-1; sending in a 0 will move 1 byte,
sending a 0FFFFh will move 64k.


; IN: DX:AX = segment/offset address of memory area
;
;OUT: DH = Page (0-F) (DL is destroyed)
; AX = Offset

PROC MakePage
push bx

mov bl,dh
shr bl,4 ;isolate upper 4 bits of segment

shl dx,4 ;make segment into ABS address
add ax,dx ;add the offset and put it in AX
adc bl,0 ;complete the addition

mov dh,bl ;put the PAGE where it goes

pop bx ; DH:AX is now the PAGE:OFFSET address
ret
ENDP


Programming DMA channels 0 thru 3

There are 3 ports that are DMA channel specific:

1) The Page register
2) The DMA count (length) register
3) The memory address (offset register)

They are as follows:

DMACH PAGE ADDRESS LENGTH

0 87h 0 1

1 83h 2 3

2 81h 4 5

3 82h 6 7


And now some general registers:

DMA Mask Register: 0Ah

bit 7 - 3 = 0 Reserved

bit 2 = 0 clear mask
= 1 set mask

bits 1 - 0 = 00 Select channel 0
= 01 select channel 1
= 10 select channel 2
= 11 select channel 3

USE: You must set the mask of the channel before you
can reprogram it.

DMA Mode Register: 0Bh

bit 7 - 6 = 00 Demand mode
= 01 Signal mode
= 10 Block mode
= 11 Cascade mode

bit 5 - 4 = 0 Reserved

bit 3 - 2 = 00 Verify operation
= 01 Write operation
= 10 Read operation
= 11 Reserved

bits 1 - 0 = 00 Select channel 0
= 01 select channel 1
= 10 select channel 2
= 11 select channel 3

USE: Tell the DMAC what to do. Common modes are:

48h (Read operation, Signal mode)
Used to read data from host memory and send to whomever
polls it.

44h (Write operation, Signal mode)
Used to write data taken from a device to memory.

DMA clear byte ptr: 0Ch

USE: Send a zero to reset the internal ptrs



WHAT TO DO:


1) Set the Mask bit for the channel

mov al,4
add al,[DMA_Channel]
out 0ah,al

2) Clear Byte Ptr

sub al,al
out 0Ch,al

3) Set the DMA transfer mode

mov al,48h ;MODE output (read)
add al,[DMA_Channel]
out 0Bh,al

4) Set the memory ADDRESS and LENGTH

; AX = offset
; CX = Length
;[DMA_Base] = port # of memory address

mov dx,[DMA_Base]
out dx,al ;send lower byte address
mov al,ah
out dx,al ;send high byte address

inc dl ;point to Count port
mov al,cl
out dx,al ;send low byte length
mov al,ch
out dx,al ;send high byte length

5) Set the DMA page

; AL = Page

mov dx,[Dma_Page]
out dx,al ; write the Page

6) Clear DMA mask bit

mov al,[byte DMA_Channel]
out 0Ah,al ; port 0Ah, DMA-1 mask reg bit

7) Program the other device that is going to use the DMA output/input



; This routine programs the DMAC for channels 0-3
;
; IN: [DMA_Channel], [DMAbaseAdd], [DMApageReg] must be setup
; [DAMBaseAdd] = Memory Address port
;
; dh = mode
; ax = address
; cx = length
; dl = page

PROC Prog_DMA03 NEAR
push bx
mov bx,ax

mov al,4
add al,[DMA_Channel]
out 0Ah,al ; mask reg bit

sub al,al
out 0Ch,al ; clr byte ptr

mov al,dh
add al,[DMA_Channel]
out 0Bh,al ; set mode reg

push dx

mov dx,[DMAbaseAdd]
mov al,bl
out dx,al ; set base address low
mov al,bh
out dx,al ; set base address high

inc dx ;point to length
mov al,cl
out dx,al ; set length low
mov al,ch
out dx,al ; set length high

pop dx

mov al,dl
mov dx,[DmaPageReg]
out dx,al ; set DMA page reg

mov al,[DMA_Channel]
out 0Ah,al ; unmask (activate) dma channel
pop bx
ret
ENDP



Programming DMA channels 4 thru 7



Again, there are 3 ports that are DMA channel specific:

1) The Page register
2) The DMA count (length) register
3) The memory address (offset register

They are as follows:

DMACH PAGE ADDRESS LENGTH

4 8Fh C0h C2h

5 8Bh C4h C6h

6 89h C8h CAh

7 8Ah CCh CEh


And now some general registers:

DMA Mask Register: 0D4h

bit 7 - 3 = 0 Reserved

bit 2 = 0 clear mask
= 1 set mask

bits 1 - 0 = 00 Select channel 4
= 01 select channel 5
= 10 select channel 6
= 11 select channel 7

USE: You must set the mask of the channel before you
can reprogram it.

DMA Mode Register: 0D6h

bit 7 - 6 = 00 Demand mode
= 01 Signal mode
= 10 Block mode
= 11 Cascade mode

bit 5 - 4 = 0 Reserved

bit 3 - 2 = 00 Verify operation
= 01 Write operation
= 10 Read operation
= 11 Reserved

bits 1 - 0 = 00 Select channel 4
= 01 select channel 5
= 10 select channel 6
= 11 select channel 7

USE: Tell the DMAC what to do. Common modes are:

48h (Read operation, Signal mode)
Used to read data from host memory and send to whomever
polls it.

44h (Write operation, Signal mode)
Used to write data taken from a device to memory.

DMA clear byte ptr: 0D8h

USE: Send a zero to reset the internal ptrs


WHAT TO DO: (exactly the same thing, just different io PORTs)


1) Set the Mask bit for the channel

mov al,[DMA_Channel] ;because the DMA's are 4-7, bit #3
out 0D4h,al ; is already set

2) Clear Byte Ptr

sub al,al
out 0D8h,al

3) Set the DMA transfer mode

mov al,[DMA_Channel]
sub al,4
or al,48h ;MODE output (read)
out 0D6h,al

4) Set the memory ADDRESS and LENGTH

; AX = offset
; CX = Length
;[DMA_Base] = port # of memory address

mov dx,[DMA_Base]
out dx,al ;send lower byte address
mov al,ah
out dx,al ;send high byte address

add dl,2 ;point to Count port (seperated by 2)
mov al,cl
out dx,al ;send low byte length
mov al,ch
out dx,al ;send high byte length

5) Set the DMA page

; AL = Page

mov dx,[Dma_Page]
out dx,al ; write the Page

6) Clear DMA mask bit

mov al,[byte DMA_Channel]
and al,00000011b
out 0d4h,al ; port 0Ah, DMA-1 mask reg bit

7) Program the other device that is going to use the DMA output/input



; This routine programs the DMAC for channels 4-7
;
; IN: [DMA_Channel], [DMAbaseAdd], [DMApageReg] must be setup
; [DAMBaseAdd] = Memory Address port
;
; dh = mode
; ax = address
; cx = length
; dl = page

PROC Prog_DMA47 NEAR
push bx
mov bx,ax

mov al,[DMA_Channel]
out 0D4h,al ; mask reg bit

sub al,al
out 0D8h,al ; clr byte ptr

mov al,[DMA_Channel]
sub al,4
add al,dh
out 0D6h,al ; set mode reg

push dx

mov dx,[DMAbaseAdd]
mov al,bl
out dx,al ; set base address low
mov al,bh
out dx,al ; set base address high

add dl,2 ;point to length
mov al,cl
out dx,al ; set length low
mov al,ch
out dx,al ; set length high

pop dx

mov al,dl
mov dx,[DmaPageReg]
out dx,al ; set DMA page reg

mov al,[DMA_Channel]
and al,00000011b
out 0D4h,al ; unmask (activate) dma channel
pop bx
ret
ENDP


; This routine programs the DMAC for channels 0-7
;
; IN: [DMA_Channel], [DMAbaseAdd], [DMApageReg] must be setup
; [DAMBaseAdd] = Memory Address port
;
; dh = mode
; ax = address
; cx = length
; dl = page

PROC Prog_DMA NEAR
push bx
mov bx,ax

cmp [DMA_Channel],4
jb @@DoDMA03

mov al,[DMA_Channel]
out 0D4h,al ; mask reg bit

sub al,al
out 0D8h,al ; clr byte ptr

mov al,[DMA_Channel]
sub al,4
add al,dh
out 0D6h,al ; set mode reg

push dx

mov dx,[DMAbaseAdd]
mov al,bl
out dx,al ; set base address low
mov al,bh
out dx,al ; set base address high

add dl,2 ;point to length
mov al,cl
out dx,al ; set length low
mov al,ch
out dx,al ; set length high

pop dx

mov al,dl
mov dx,[DmaPageReg]
out dx,al ; set DMA page reg

mov al,[DMA_Channel]
and al,00000011b
out 0D4h,al ; unmask (activate) dma channel
pop bx
ret

@@DoDMA03:
mov al,4
add al,[DMA_Channel]
out 0Ah,al ; mask reg bit

sub al,al
out 0Ch,al ; clr byte ptr

mov al,dh
add al,[DMA_Channel]
out 0Bh,al ; set mode reg

push dx

mov dx,[DMAbaseAdd]
mov al,bl
out dx,al ; set base address low
mov al,bh
out dx,al ; set base address high

inc dx ;point to length
mov al,cl
out dx,al ; set length low
mov al,ch
out dx,al ; set length high

pop dx

mov al,dl
mov dx,[DmaPageReg]
out dx,al ; set DMA page reg

mov al,[DMA_Channel]
out 0Ah,al ; unmask (activate) dma channel
pop bx
ret
ENDP