at91cap7e ATMEL Corporation, at91cap7e Datasheet - Page 57

• Manufacturer Part Number: at91cap7e
• Description: Customizable Microcontroller
• Manufacturer: ATMEL Corporation

12.2.4.1
12.2.4.2
12.2.4.3
8549A–CAP–10/08
Modes and Exception Handling
Status Registers
Exception Types
Registers R0 to R7 are unbanked registers. This means that each of them refers to the same 32-
bit physical register in all processor modes. They are general-purpose registers, with no special
uses managed by the architecture, and can be used wherever an instruction allows a general-
purpose register to be specified.
Registers R8 to R14 are banked registers. This means that each of them depends on the current
mode of the processor.
All exceptions have banked registers for R14 and R13.
After an exception, R14 holds the return address for exception processing. This address is used
to return after the exception is processed, as well as to address the instruction that caused the
exception.
R13 is banked across exception modes to provide each exception handler with a private stack
pointer.
The fast interrupt mode also banks registers 8 to 12 so that interrupt processing can begin with-
out having to save these registers.
A seventh processing mode, System Mode, does not have any banked registers. It uses the
User Mode registers. System Mode runs tasks that require a privileged processor mode and
allows them to invoke all classes of exceptions.
Exception vectors are located starting at address 0x0000 0000.
All other processor states are held in status registers. The current operating processor status is
in the Current Program Status Register (CPSR). The CPSR holds:
All five exception modes also have a Saved Program Status Register (SPSR) that holds the
CPSR of the task immediately preceding the exception.
The ARM7TDMI
The types of exceptions are:
Exceptions are generated by internal and external sources.
More than one exception can occur in the same time.
When an exception occurs, the banked version of R14 and the SPSR for the exception mode
are used to save state.
• four ALU flags (Negative, Zero, Carry, and Overflow)
• two interrupt disable bits (one for each type of interrupt)
• one bit to indicate ARM or Thumb execution
• five bits to encode the current processor mode
• fast interrupt (FIQ)
• normal interrupt (IRQ)
• memory aborts (used to implement memory protection or virtual memory)
• attempted execution of an undefined instruction
• software interrupts (SWIs)
supports five types of exception and a privileged processing mode for each type.
AT91CAP7E
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