Port Address
一般而言，COM口的I/O地址是固定的，
3F8/2F8 BaseAdd + 0 Transmit/Receive Buffer
3F9/2F9 BaseAdd + 1 IER
3FA/2FA BaseAdd + 2 IIR(Read)/FCR(write)
3FB/2FB BaseAdd + 3 LCR
3FC/2FC BaseAdd + 4 MCR
3FD/2FD BaseAdd + 5 LSR
3FE/2FE BaseAdd + 6 MSR
3FF/2FF BaseAdd + 7 Scratch Pad Register
Abbreviate
CD - Carrier Detect
CTS - Clear to Send
DCD - Data Carrier Detect
DCE - Data communication Equipment
DDCD - Delta Data Carrier Detect
DLAB - baud rate divisor
DSR - Data set ready
DTE - Data Terminal Equipment
DTR - Data terminal ready
FCR - FIFO Control Register
FIFO - First In First Out
IER - Interrupt Enable Register
IIR - Interrupt Identification Register
LCR - Line Control Register
LSR - Line Status Register
MCR - Modem Control Register
MSR - Modem Status Register
RD - Receive Data
RI - Ring indicator
RTS - Request to Send
TD - Transmit Data
THRE - Transmitter Holding Register Empty
TSRE - Transmitter Shift Register Empty
Set Baud Rate
当LCR(Base Address+3) BIT7是1的时候，往Base+0/Base+1这两个端口写的数据就是要设置的波特率。
Baud Rate
Base+1
Base+0
Baud Rate
Base+1
Base+0
50
0x09
0x00
2400
0x00
0x30
110
0x04
0x17
3600
0x00
0x20
150
0x03
0x00
4800
0x00
0x18
300
0x01
0x80
7200
0x00
0x10
600
0x00
0xC0
9600
0x00
0x0C
1200
0x00
0x60
19200
0x00
0x06
1800
0x00
0x40
38400
0x00
0x03
2000
0x00
0x3A
57600
0x00
0x02
115200
0x00
0x01
Example Code:
BaudRateTable:
dw 01h ; 115200 0
dw 02h ; 57600 1
dw 03h ; 38400 2
dw 06h ; 19200 3
dw 0Ch ; 9600 4
mov dx, Base+3
in al, dx
jmp short $+2 ; delay
or al, 80h
out dx, al ; set LCR BIT7
jmp short $+2
push ax ; save the data
lea si, cs:BaudRateTable
shl bx, 1 ; bx =0/1/2/3/4
add si, bx ; si point the divisor
mov ax, word ptr cs:[si]
; al, base+0 value
; ah, base+1 value
mov dx, Base+0
out dx, al
jmp short $+2
xchg ah, al
inc dx
out dx, al
jmp short $+2
pop ax
and al, 07fh ; clear BIT7
mov dx, base+3
out dx, al
jmp short $+2