Channel 0 generates IRQ0 at a programmable rate. Write mode/control to port 0x43, then latch low/high bytes of divisor to port 0x40. For example, divisor 1193182 / 100 yields roughly 100 Hz ticks for a scheduler heartbeat.
Frequency formula
nasm
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Input clock: ~1.193182 MHz on PC/AT
Mode 3: Square wave generator (common for timer interrupts)
Read back: Latch count before reprogramming during run
Pair PIT with PIC EOI in your timer ISR
PIT integration notes
The PIT output feeds IRQ0 on the master PIC. Until you remap the PIC, the default vector may conflict with CPU exceptions, so bring up remapping before unmasking IRQ0. If the divisor is programmed while the channel is running, latch the counter first to avoid glitches. Many hobby kernels use the PIT as the first periodic interrupt source because it needs no device-specific driver beyond I/O ports.
Why for this exercise
You will program the PIT divisor and verify IRQ0 fires at the expected rate. This exercise requires port 0x43/0x40 sequencing and connecting the interrupt to your existing ISR.
Compute 8253/8254 PIT programming values in C from stdin requests.
Input format:
Line 1: N, the number of commands.
Then one command per line:
"FREQ <hz>": wants IRQ0 at hz Hz
"DIV <d>": divisor d (1-65535) already programmed
"TICKS <d> <t>": t ticks elapsed at the rate divisor d produces
Requirements:
The PIT input clock is 1193182 Hz; divisor is 16-bit (1-65535).
FREQ <hz>: divisor = 1193182 / hz (integer division). If the divisor falls outside 1-65535 print "FREQ <hz> -> error divisor=<d> out of range".
For a valid divisor print: "FREQ <hz> -> divisor=<d> cmd=0x36 low=0x<hh> high=0x<hh> actual=<f>Hz period=<f>ms" where cmd 0x36 = channel 0 | lobyte/hibyte | mode 3 | binary, low/high are the bytes written to port 0x40, actual = 1193182 / d, period = ms per tick (both formatted %.2f).
DIV <d>: same fields as FREQ (with "DIV <d> -> " prefix); invalid divisor prints "DIV <d> -> error divisor out of range".
TICKS <d> <t>: print "TICKS <d> <t> -> elapsed=<f>ms" where elapsed = t * d * 1000 / 1193182 (%.2f).