AT91SAM7X512 datasheet(40/687 Pages) ATMEL

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6120H–ATARM–17-Feb-09

AT91SAM7X512/256/128 Preliminary

11.2

ARM7TDMI Processor

For further details on ARM7TDMI, refer to the following ARM documents:

ARM Architecture Reference Manual (DDI 0100E)

ARM7TDMI Technical Reference Manual (DDI 0210B)

11.2.1

Instruction Type

Instructions are either 32 bits long (in ARM state) or 16 bits long (in THUMB state).

11.2.2

Data Type

ARM7TDMI supports byte (8-bit), half-word (16-bit) and word (32-bit) data types. Words must be

aligned to four-byte boundaries and half words to two-byte boundaries.

Unaligned data access behavior depends on which instruction is used where.

11.2.3

ARM7TDMI Operating Mode

The ARM7TDMI, based on ARM architecture v4T, supports seven processor modes:

User: The normal ARM program execution state

FIQ: Designed to support high-speed data transfer or channel process

IRQ: Used for general-purpose interrupt handling

Supervisor: Protected mode for the operating system

Abort mode: Implements virtual memory and/or memory protection

System: A privileged user mode for the operating system

Undefined: Supports software emulation of hardware coprocessors

Mode changes may be made under software control, or may be brought about by external inter-

rupts or exception processing. Most application programs execute in User mode. The non-user

modes, or privileged modes, are entered in order to service interrupts or exceptions, or to

access protected resources.

11.2.4

ARM7TDMI Registers

The ARM7TDMI processor has a total of 37registers:

• 31 general-purpose 32-bit registers

• 6 status registers

These registers are not accessible at the same time. The processor state and operating mode

determine which registers are available to the programmer.

At any one time 16 registers are visible to the user. The remainder are synonyms used to speed

up exception processing.

relative to the current instruction.

R14 holds the return address after a subroutine call.

R13 is used (by software convention) as a stack pointer.

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