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===Introduction===
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DSDT stands for ''Differentiated System Description Table''. It Is a major [[ACPI]] table and is used to describe what peripherals the machine has. Also holds information on PCI IRQ mappings and power management. For example when powering down by the OS,
===How hardware manufacturers generate it===
You can use an ASL (ACPI Source Language) compiler to generate DSDT AML (ACPI Machine Language) bytecode from a C-like language. To get an idea, take a glimpse on bochs source:
* bios/acpi-dsdt.dsl for human readable source
* bios/acpi-dsdt.hex for hex dump of the bytecode output
For example here's the description of the [[RTC|Real Time Clock]]:
<source lang="c">
/* PIIX3 ISA bridge */
Device (ISA) {
Name (_ADR, 0x00010000)
/* PIIX PCI to ISA irq remapping */
OperationRegion (P40C, PCI_Config, 0x60, 0x04)
/* Real-time clock */
Device (RTC)
{
Name (_HID, EisaId ("PNP0B00"))
Name (_CRS, ResourceTemplate ()
{
IO (Decode16, 0x0070, 0x0070, 0x10, 0x02)
IRQNoFlags () {8}
IO (Decode16, 0x0072, 0x0072, 0x02, 0x06)
})
}
</source>
and what's it became after compilation:
<pre>
0x00,0xA4,0x00,0x5B,0x82,0x42,0x23,0x49, /* 00000270 "...[.B#I" */
0x53,0x41,0x5F,0x08,0x5F,0x41,0x44,0x52, /* 00000278 "SA_._ADR" */
0x0C,0x00,0x00,0x01,0x00,0x5B,0x80,0x50, /* 00000280 ".....[.P" */
0x34,0x30,0x43,0x02,0x0A,0x60,0x0A,0x04, /* 00000288 "40C..`.." */
0x5B,0x82,0x2D,0x52,0x54,0x43,0x5F,0x08, /* 00000290 "[.-RTC_." */
0x5F,0x48,0x49,0x44,0x0C,0x41,0xD0,0x0B, /* 00000298 "_HID.A.." */
0x00,0x08,0x5F,0x43,0x52,0x53,0x11,0x18, /* 000002A0 ".._CRS.." */
0x0A,0x15,0x47,0x01,0x70,0x00,0x70,0x00, /* 000002A8 "..G.p.p." */
0x10,0x02,0x22,0x00,0x01,0x47,0x01,0x72, /* 000002B0 ".."..G.r" */
0x00,0x72,0x00,0x02,0x06,0x79,0x00,0x5B, /* 000002B8 ".r...y.[" */
0x82,0x44,0x04,0x4B,0x42,0x44,0x5F,0x08, /* 000002C0 ".D.KBD_." */
</pre>
Most likely you will never generate such a binary. But it helps if you know it's source form when you came to decode.
===Purpose of DSDT===
When your OS boots, it should parse the memory for [[ACPI]] tables. Then locate DSDT (and other tables as well, like [[FADT]]), and decode it to get the list of installed devices. If you have that list, it's rather easy to load [[device driver]] for each.
Also note that there are buggy tables, so you should always kept the possibility to load DSDT data from a user provided file instead. This file could be located in your [[initrd|initial ramdisk]], loaded with your kernel along on boot. That would solve the chicken-egg problem of loading the DSDT file from a device that's IO addresses is defined in the DSDT.
==See Also==
===ACPI===
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