Reset vector

In computing, the reset vector is the default location a central processing unit will go to find the first instruction it will execute after a reset. The reset vector is a pointer or address, where the CPU should always begin as soon as it is able to execute instructions. The address is in a section of non-volatile memory initialized to contain instructions to start the operation of the CPU, as the first step in the process of booting the system containing the CPU.

Processors

• The reset vector for the 8086 processor is at physical address FFFF0h (16 bytes below 1 MB). The value of the CS register at reset is FFFFh and the value of the IP register at reset is 0000h to form the segmented address FFFFh:0000h, which maps to physical address FFFF0h.^[1]
• The reset vector for the 80286 processor is at physical address FFFFF0h (16 bytes below 16 MB). The value of the CS register at reset is F000h with the descriptor base set to FF0000h and the value of the IP register at reset is FFF0h to form the segmented address FF000h:FFF0h, which maps to physical address FFFFF0h in real mode.^[2] This was changed to allow sufficient space to switch to protected mode without modifying the CS register.^[3]
• The reset vector for the 80386 and later x86 processors is physical address FFFFFFF0h (16 bytes below 4 GB). The value of the selector portion of the CS register at reset is F000h, the value of the base portion of the CS register is FFFF0000h, and the value of the IP register at reset is FFF0h^[4] to form the segmented address FFFF0000h:FFF0h, which maps to the physical address FFFFFFF0h in real mode.^[5][6]
• The reset vector for PowerPC/Power ISA processors is at an effective address of 0x00000100 for 32-bit processors and 0x0000000000000100 for 64-bit processors.
• The reset vector for m68k Architecture processors is 0x0 for Initial Interrupt Stack Register (IISR; Not really a reset vector and is used to initialize the stack pointer after reset.) and 0x4 for initial program counter (reset).^[7]
• The reset vector for SPARC version 8 processors is at an address of 0x00;^[8] the reset vector for SPARC version 9 processors is at an address of 0x20 for power-on reset, 0x40 for watchdog reset, 0x60 for externally initiated reset, and 0x80 for software-initiated reset.^[9]
• The reset vector for MIPS32 processors is at virtual address 0xBFC00000,^[10] which is located in the last 4 Mbytes of the KSEG1 non-cacheable region of memory.^[11] The core enters kernel mode both at reset and when an exception is recognized, hence able to map the virtual address to physical address.^[12]
• The reset vector for the ARM family of processors is address 0x0^[13] or 0xFFFF0000. During normal execution RAM is re-mapped to this location to improve performance, compared to the original ROM-based vector table.^[14]

See also

• Booting
• Reboot (computing)
• Control-Alt-Delete
• Power-on self-test
• Hardware reset

References

1. "iAPX 86,88 User's Manual" (PDF). Intel. 1981. System Reset, p. 2-29, table 2-4. Retrieved April 15, 2018.
2. "AMD 80286 Datasheet" (PDF). AMD. 1985. p. 13. the 286 begins execution in real mode with the instruction at physical location FFFFF0H.
3. "iAPX 286 Programmer's Reference Manual" (PDF). Intel. 1983. Appendix D, iAPX 86/88 Software Compatibility Considerations, p. D-2. Retrieved April 15, 2018. After reset, CS:IP = F000:FFF0 on the iAPX 286. This change was made to allow sufficient code space to enter protected mode without reloading CS.
4. "80386 Programmer's Reference Manual" (PDF). Intel. 1990. Section 10.1 Processor State After Reset, pages 10-1 - 10.3.
5. "80386 Programmer's Reference Manual" (PDF). Intel. 1990. Section 10.2.3 First Instruction, p. 10-4. Retrieved November 3, 2013. Execution begins with the instruction addressed by the initial contents of the CS and IP registers. To allow the initialization software to be placed in a ROM at the top of the address space, the high 12 bits of addresses issued for the code segment are set, until the first instruction which loads the CS register, such as a far jump or call. As a result, instruction fetching begins from address 0FFFFFFF0H.
6. "Intel® 64 and IA-32 Architectures Software Developer's Manual" (PDF). Intel. May 2012. Section 9.1.4 First Instruction Executed, p. 2611. Retrieved August 23, 2012. The first instruction that is fetched and executed following a hardware reset is located at physical address FFFFFFF0h. This address is 16 bytes below the processor’s uppermost physical address. The EPROM containing the software-initialization code must be located at this address.
7. Labrosse, Jean J. (2008). Embedded Software. Newnes. ISBN 9780750685832.
8. The SPARC Architecture Manual, Version 8. SPARC International. p. 75.
9. The SPARC Architecture Manual, Version 9. SPARC International. pp. 109–112.
10. "MIPS32 Architecture For Programmers; Vol III: The MIPS32 Privileged Resource Architecture" (PDF). MIPS Technologies.
11. Noergaard, Tammy (2005-02-28). Embedded Systems Architecture: A Comprehensive Guide for Engineers and Programmers. Elsevier. ISBN 9780080491240.
12. (PDF). cdn2.imgtec.com. August 29, 2008. Archived from the original (PDF) on 2017-08-26.
13. "5.9.1. Vector Table and Reset". Cortex-M3 Technical Reference Manual. Retrieved 2017-11-10.
14. "Boot sequence for an ARM based embedded system -2 - DM". www.embeddedrelated.com. Retrieved 2017-11-10.