Serial Programming - upload.wikimedia....

Finding the Port I/O Address for the UART
4.3.1 Looking up UART Base Address in RAM
We will get back to the issue of the IRQ Number in a little bit, but for now we need to know
where to start accessing information about each UART. As demonstrated previously, DOS
also keeps track of where the UART IO ports are located at for its own purpose, so you can
try to "look up" within the memory tables that DOS uses to try and find the correct address
as well. This doesn't always work, because we are going outside of the normal DOS API
structure. Alternative operating systems ( FreeDOS works fine here ) that are otherwise
compatible with MS-DOS may not work in this manner, so take note that this may simply
give you a wrong result altogether.
The addresses for the serial I/O Ports can be found at the following locations in RAM:
Port Segment Offset
COM1 $0040 $0000
COM2 $0040 $0002
COM3 $0040 $0004
COM4 $0040 $0006
Those addresses are written to memory by the BIOS when it boots. If one of the ports
doesn't exist, the BIOS writes zero to the respective address. Note that the addresses are
given in segment:offset format and that you have to multiply the address of the segment
with 16 and add the offset to get to the physical address in memory. This is where DOS
"finds" the port addresses so you can run the first sample program in this chapter.
In assembler you can get the addresses like this:
; Data Segment
.data
Port dw 0
...
; Code Segment
.code
mov ax,40h
mov es,ax
mov si,0
mov bx,Port ; 0 - COM1 , 1 - COM2 ...
shl bx,1
mov Port, es:[si+bx]
In Turbo Pascal, you can get at these addresses almost the same way and in some ways
even easier because it is a "high level language". All you have to do is add the following line
to access the COM Port location as a simple array:
var
ComPort: array [1..4] of Word absolute $0040:$0000;
The reserved, non standard, word absolute is a flag to the compiler that instead of
"allocating" memory, that you already have a place in mind to have the computer look
instead. This is something that should seldom be done by a programmer unless you are
accessing things like these I/O port addresses that are always stored in this memory location.
57