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Serial Port Console Redirection Table (SPCR)

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This document defines the content of the Serial Port Console Redirection Table. This table is used to indicate whether a serial port or a non-legacy UART interface is available for use with Microsoft® Windows® Emergency Management Services (EMS).

The table provides information about the configuration and use of the serial port or non-legacy UART interface. On a system where the BIOS or system firmware uses the serial port for console input/output, this table should be used to convey information about the settings, to ensure a seamless transition between the firmware console output and Windows EMS output.

This table must be located in system memory with other ACPI tables, and it must be referenced in the ACPI RSDT table.

Patent Notice: Microsoft is making certain patent rights available for implementations of this specification under two options:

  1. Microsoft’s Community Promise, available at https://www.microsoft.com/openspecifications/en/us/programs/community-promise/default.aspx; or
  2. The Open Web Foundation Final Specification Agreement Version 1.0 ("OWF 1.0") as of October 1, 2012, available on the Open Web Foundation web site.
Field Byte Length Byte Offset Description
Header
Signature 4 0 'SPCR'. Signature for the Serial Port Console Redirection Table.
Length 4 4 Length, in bytes, of the entire Serial Port Console Redirection Table, including NamespaceString.
Revision 1 8 The current table revision is 4.
Checksum 1 9 Entire table must sum to zero.
OEM ID 6 10 Original equipment manufacturer (OEM) ID.
OEM Table ID 8 16 For the Serial Port Console Redirection Table, the table ID is the manufacturer model ID.
OEM Revision 4 24 OEM revision of Serial Port Console Redirection Table for supplied OEM Table ID.
Creator ID 4 28 Vendor ID of utility that created the table.
Creator Revision 4 32 Revision of utility that created the table.
Interface Type 1 36 Indicates the type of the register interface:
For Revision 1:
  • 0 = Full 16550 interface
  • 1 = Full 16450 interface (must also accept writing to the 16550 FCR register)
  • 2-255 = Reserved
For Revision 2 or higher:
See the Serial Port Subtypes in Table 3 of the DBG2 Specification.
Reserved 3 37 Must be 0.
Base Address 12 40 The base address of the Serial Port register set described using the ACPI Generic Address Structure, or 0 if console redirection is disabled.

Note:
COM1 (0x3F8) would be:
  • Integer Form: 0x 01 08 00 00 00000000000003F8
  • Viewed in Memory: 0x01080000F803000000000000
COM2 (0x2F8) would be:
  • Integer Form: 0x 01 08 00 00 00000000000002F8
  • Viewed in Memory: 0x01080000F802000000000000
Interrupt Type 1 52 Interrupt type(s) used by the UART:
  • Bit[0]: PC-AT-compatible dual-8259 IRQ interrupt
  • Bit[1]: I/O APIC interrupt (Global System Interrupt)
  • Bit[2]: I/O SAPIC interrupt (Global System Interrupt)
  • Bit[3]: ARMH GIC interrupt (Global System Interrupt)
  • Bit[4]: RISC-V PLIC/APLIC interrupt (Global System Interrupt)
  • Bit[5:7]: Reserved (must be set to 0)
Where
  • 0 = Not supported
  • 1 = Supported
Set this field to zero if the described interface only supports polled operation.
Platforms with both a dual-8259 and an I/O APIC or I/O SAPIC must set the IRQ bit (Bit[0]) and the corresponding Global System Interrupt bit (e.g. a system that supported 8259 and SAPIC would be 5).
IRQ 1 53 The PC-AT-compatible IRQ used by the UART:
  • 2-7, 9-12, 14-15 = Valid IRQs respectively
  • 0-1, 8, 13, 16-255 = Reserved
Valid only if Bit[0] of the Interrupt Type field is set.
Global System Interrupt 4 54 The Global System Interrupt (GSIV) used by the UART.
Not valid if Bit[1:7] of the Interrupt Type field is 0.
If bit 3 of the Interrupt Type field is set (ARMH GIC) then an Arm GIC interrupt controller is used. Arm GIC SGI and PPI interrupts cannot be used for the UART, so it is forbidden for this field to be set to any value in {0, ..., 31} or in {1056, ..., 1119}.
Configured Baud Rate 1 58 The baud rate the BIOS used for redirection:
  • 0 = As is, operating system relies on the current configuration of serial port until the full featured driver will be initialized.
  • 3 = 9600
  • 4 = 19200
  • 6 = 57600
  • 7 = 115200
  • 1-2, 5, 8-255 = Reserved
Parity 1 59
  • 0 = No Parity
  • 1-255 = Reserved
Stop Bits 1 60
  • 1 = 1 Stop bit
  • 0, 2-255 = Reserved
Flow Control 1 61
  • Bit[0]: DCD required for transmit
  • Bit[1]: RTS/CTS hardware flow control
  • Bit[2]: XON/XOFF software control
  • Bit[3:7]: Reserved, must be 0
Terminal Type 1 62 The terminal protocol the BIOS was using for console redirection:
  • 0 = VT100
  • 1 = Extended VT100 (VT100+)
  • 2 = VT-UTF8
  • 3 = ANSI
  • 4-255 = Reserved
Language 1 63 Language which the BIOS was redirecting. Must be 0.
PCI Device ID 2 64 Designates the Device ID of a PCI device that contains a UART to be used as a headless port.
Must be 0xFFFF if it is not a PCI device.
PCI Vendor ID 2 66 Designates the Vendor ID of a PCI device that contains a UART to be used as a headless port.
Must be 0xFFFF if it is not a PCI device.
PCI Bus Number 1 68 PCI Bus Number if table describes a PCI device.
Must be 0x00 if it is not a PCI device.
PCI Device Number 1 69 PCI Device Number if table describes a PCI device.
Must be 0x00 if it is not a PCI device.
PCI Function Number 1 70 PCI Function Number if table describes a PCI device.
Must be 0x00 if it is not a PCI device.
PCI Flags 4 71 PCI Compatibility flags bitmask. Should be zero by default.
  • Bit[0]: Operating System should NOT suppress PNP device enumeration or disable power management for this device. Must be 0 if it is not a PCI device.
  • Bit[1-31]: Reserved, must be 0.
PCI Segment 1 75 PCI segment number.

For systems with fewer than 255 PCI buses, this number must be 0.
UART Clock Frequency 4 76 For Revision 2 or lower:
  • Must be 0.
For Revision 3 or higher:
  • Zero, indicating that the UART clock frequency is indeterminate.
  • A non-zero value indicating the UART clock frequency in Hz.
Precise Baud Rate 4 80 Contains a specific non-zero baud rate which overrides the value of the Configured Baud Rate field. If this field is zero or not present, Configured Baud Rate is used. See note below.
NamespaceStringLength 2 84 Length, in bytes, of NamespaceString, including NUL characters.
NamespaceStringOffset 2 86 Offset, in bytes, from the beginning of this structure to the field NamespaceString[]. This value must be valid because this string must be present.
NamespaceString[] NamespaceStringLength NamespaceStringOffset NUL-terminated ASCII string to uniquely identify this device. This string consists of a fully qualified reference to the object that represents this device in the ACPI namespace. If no namespace device exists, NamespaceString[] must only contain a single '.' (ASCII period) character.

Note on the Baud Rate Fields

The Configured Baud Rate field has existed as a single-byte field since the creation of the SPCR table and is widely supported by operating systems. However, because it is an enumeration, it is limited in its ability to precisely describe non-traditional baud rates, such as those used by high speed UARTs. Thus, the Precise Baud Rate field was added to enable firmware to provide supporting operating systems a DWORD value which describes a specific baud rate (e.g. 1500000). When the Precise Baud Rate field contains a non-zero value, the Configured Baud Rate field shall be zero.

Revision History

Date Rev Description
2/15/00 .10 Created
3/1/00 .50 ‘SPCR’. Signature Data added
3/20/00 .55 Data revised to include port and irq
3/22/00 .56 Clarified port identification Added ability to disable redirection. Added pointer to the Generic Register Address Structure
3/23/00 .56a Formatting, disclaimer, copy editing
4/24/00 .6 Posted on web for WinHEC
4/24/00 .6 Public review draft published
5/25/00 .61 Correction to BASE_ADDRESS description
5/25/00 .61 Public review draft published
5/31/00 .7 Correction to BASE_ADDRESS description examples. Added 16540 interface.
5/31/00 .71 Changed the info on the GRAS from a note to a “*”
5/31/00 .71 Public review draft published
6/1/00 .72 Changed GRAS COM port examples to be little-endian. Added text to the end of the line
7/12/00 .75 Fixed IRQ Description. Fixed various format issues Added PCI bus information.
7/26/00 .76 Update to PCI field name “Device Number”. Changed intro language to include non-legacy UART.
8/10/00 .77 Changed interrupt information, adding APIC and SAPIC Added flow control
9/22/00 .78 Added PCI Segment
10/25/00 .80 Fixed PCI Flags section. Added Terminal Types Added 16450 FCR info
10/1/01 .95 Removed language codes
1/11/02 1.00 adding updated licensing spec to 1.00
3/12/14 1.01 Released under Microsoft Community Promise
6/2/14 1.02 Changed Table Revision to 2 and added support for additional Interface Types, as defined in the DBG2 specification.
8/10/15 1.03 Updated patent notice.
7/23/2018 1.04
6/5/2020 1.05 Edited formatting
9/1/2020 1.06 Edited formatting and updated link to DBG2 specification
2/17/2021 1.07 Fixed incorrect description in Stop Bits field. Undo accidental removal of Flow Control field. Edited formatting.
10/7/2021 1.08 Changed Table Revision to 3 and created field for UART Clock Frequency. Edited formatting.
4/10/2023 1.09 Changed Table Revision to 4. Added RISC-V and polled support to Interrupt Type field. Added new fields: Precise Baud Rate, NamespaceStringLength, NamespaceStringOffset, and NamespaceString[].
5/1/2023 1.10 Clarified wording in Global System Interrupt and UART Clock Frequency fields.