COM1 at I/O base 0x3F8 uses a 16550 UART with FIFOs and line status register (LSR). Poll LSR bit 5 (THRE) before writing transmit holding register, or enable receive interrupts via IER. For example, sending 'H' means waiting for THRE then out 0x3F8, al.
Init and transmit
nasm
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DLAB: Divisor latch access bit in LCR
IER: Bit 0 enables received-data interrupt
QEMU redirects COM1 to stdout with -serial stdio
FIFO reset via FCR improves throughput
QEMU serial wiring
-serial stdio merges COM1 with the terminal so polled output appears instantly. Divisor 1 with 115200 baud is the common lab default when the input clock is 1.8432 MHz. Enable FIFOs via FCR bit 0 to reduce overrun during bursts. Line status bit 0 indicates received data ready for blocking reads without interrupts.
Why for this exercise
You will initialize COM1 and poll (or interrupt-drive) character output. This exercise requires baud divisor setup and LSR polling before each transmitted byte.
Compute UART 16550 setup values and decode the line status register in C from stdin requests.
Input format:
Line 1: N, the number of commands.
Then one command per line:
"BAUD <rate>": wants COM1 at rate baud
"DIV <d>": divisor latch value d
"LSR <hex>": a byte read from port 0x3FD
Requirements:
The UART divides its 1.8432 MHz input clock by 16, so baud = 115200 / divisor; divisor is 16-bit.
BAUD <rate>: divisor = 115200 / rate (integer division). Valid divisors are 1-65535; otherwise print "BAUD <rate> -> error divisor=<d> out of range". Success prints "BAUD <rate> -> divisor=<d> (0x<hh>) lcr=0x03 8N1 actual=<b> baud" where actual = 115200 / divisor.
DIV <d>: prints "DIV <d> -> baud=<b> divisor=0x<hh> lcr=0x03 8N1"; out-of-range prints "DIV <d> -> error divisor out of range".
LSR <hex>: mask to 8 bits and decode bits 0-6 in order: DR (data ready), OE, PE, FE, BI, THRE, TEMT, printing "LSR 0x<hh>: <flags>" with flags separated by single spaces, or " -" if none set. Bit 7 is ignored.