;-----------------------------------------------------------------------------
; VGA status port
STATUS = 03DAh ; status register port
IS_HVSYNC = 01
IS_VSYNC = 08
;-----------------------------------------------------------------------------
; Programmable Interrupt Timer (PIT) chip 8253
;PIT data ports
PIT0 = 40h ; channel 0, its output is connected to the IRQ0 input
PIT1 = 41h ; channel 1, do nor use this, on AT was used to refresh dram
PIT2 = 42h ; channel 2, can be connected to the pc-speaker
;PIT control port
PIT_CTRL = 43h
; COMMAND WORDS (actually bytes)
;
; bit 7 6 5 4 3 2 1 0
; value counter_select read_write_latch counter_mode bcd_on
T_SET0 = 036h ; channel 0, square wave, READ/WRITE COUNT INTERVAL
; LSB then MSB on data port
; this is the standard way to reprogram the IRQ frequency
; or to read the programmed time count value
T_LATCH0 = 000h ; channel 0, LATCH counter (read current count value)
; then if the timer channel was programmed with two bytes
; first LSB, then MSB
; else
; read byte
; end if
; this is the standard way to "read on the flight"
; what's the current counter value (NOT the count interval)
T_SET2 = 0B6h ; channel 2, square wave, read/write LSB then MSB on data port
; use this to make the pc speaker buzz
; but remember to connect the channel 2 output to speaker output
;------------------------------------------------------------------------------
; Programmable Interrupt Controllers
; Master PIC: IRQ0..IRQ7 ( IRQ2 chains slave PIC)
PIC0_CTRL = 020h
PIC0_DATA = 021h
; PIC0_DATA is a irq control mask for irqs 0..7
; (if bit n is set to 1 the corresponding irq line is inhibited)
; BIT IRQ USAGE
; 0 0 timer interrupt
; 1 1 keyboard controller
; 2 2 cascade from slave PIC (PIC1) , on XT was the vretrace irq
; 3 3 com2 (usually free on single serial port systems)
; 4 4 com1
; 5 5 printer port 2 (was hard disk irq on XT systems) (usually free)
; 6 6 floppy disk
; 7 7 printer port 1 (usually free, most software uses polling)
; N.B. most of the old cards (or new cards with old software drivers)
; needs an irq ranging from 0 to 7 (because the upper irqs works
; only with 16bit cards and the irq acknowledge sequence
; is different for irq 8..15).
; Slave PIC: IRQ8..IRQF ( IRQ9 is the "alias" for the original unchained IRQ2
; found on pc-xt , so if you set a board on IRQ2 it will use IRQ9)
PIC1_CRTL = 0A0h
PIC1_DATA = 0A1h
; PIC1_DATA is a irq control mask for irqs 8..15
; (if bit n is set to 1 the corresponding irq line is inhibited)
; BIT IRQ USAGE
; 0 8 realtime clock
; 1 9 redirected vertical retrace (was irq2 on old XT systems)
; 2 10 <free>
; 3 11 <free>
; 4 12 mouse irq ( used by non-serial-port mouses)
; 5 13 coprocessor exception
; 6 14 AT hard disk irq
; 7 15 <free>
; N.B <free> means available to other expansion boards
; (i.e. sound boards, scanners or who-knows-what)
;
; to "terminate" an IRQ request on the PIC you must send to
; the control ports ....
EOI = 020h ; End Of Interrupt (activates the next irq pending on another line)
SEOI = 060h ; Specific End Of Interrupt (use this to avoid overruns)
; if there are overruns send
; (SEOI+(IRQ_number mod 8)) instead of a plain EOI
; to terminate an irq request on PIC1 you must FIRST
; send an EOI to PIC0 (because PIC1 chains PIC0 using IRQ2)
; and then terminate the irq on PIC1
