REF: PC Magizine Vol 10 No 8 Pg 178
"The 16550A UART: Breaking Old Bottlenecks"
The NS16550AF UART (Universal Asychtonous Receiver/Transmitter) contains two 16-byte FIFO (first in, first out) buffers vice the one byte register on the 8250 UART used in most machines. Systems with the newer chip may take advantage of it by activating the buffered mode as shown in the figure below. Slower machines using fast modems need these buffers, as do faster machines that attempt to run other programs in a multitasking mode while using asynchronous communications.
The buffers reduce the interrupt activity of the CPU because they can hold more data before generating an interrupt to request a transfer. This makes it easier for slower CPUs to avoid dropping characters when they receive a fast stream of data. The buffer may be set to receive 1, 4, 8, or 14 characters before the UART generates an interrupt.
Be aware that the plain 16550 (vice the 16550A) was flawed and will not work correctly if the FIFO buffers are activated. Also, some communications programs written before the introduction of the 16550A might lock up when the UART activates the previously unused upper bits of the Interrupt ID Register
JMP Start U16550A:DB ' 16550A UART $' U8250: DB ' 8250 UART $' Start: MOV DX,03F8 ; PORT ADDRESS - 0040:0000 ADD DX,+02 ; set offset to Interrupt ID Register MOV AL,C1 ; set receive buffer to 14 and enable OUT DX,AL ; change the 14 = 0x0c to 1,4,or 8 XOR AX,AX IN AL,DX PUSH AX AND AL,C0 ; set up for 16550A UART CMP AL,C0 ; is it a 16550A ? JNZ No ; no Yes: LEA DX,U16550A ; yes JMP StrOut No: LEA DX,U8250 ; print either 16550 or 8250 StrOut: MOV AH,09 INT 21 POP AX MOV AH,4C ; show the IIR setup as errorlevel INT 21 RET
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