ARM ARM7TDMI Bedienungsanleitung PDF herunterladen (Seite 22)

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Usage and Examples:

The LDM instruction permits block moves of memory to the registers and enables

efficient stack operations. The registers may be listed in any order, but the registers are

always loaded in order with the lowest numbered register getting the value form the

lowest memory address. If Rn is also listed in the register list and register writeback (W

bit) is set, the final value in Rn is unpredictable.

The addressing_mode field determines how

next_address

is calculated (bits P & W),

which control how the address is updated in conjunction with each register load. The

four addressing_mode values are;

 IA - increment address by 4 after each load (post-increment)

 IB - increment address by 4 before each load (pre-increment)

 DA - decrement address by 4 after each load (post-decrement)

 DB - decrement address by 4 before each load (pre-decrement)

The “!” following Rn controls the value of the writeback bit (bit W), and signifies that Rn

should be updated with the ending address at the end of the instruction. If the “!” is

not present (W=0), the value of Rn will be unchanged at the end of the instruction.

LDMIA R7, {R0, R2-R4} ;R0  memory[R7]

;R2  memory[R7+4]

;R3  memory[R7+8]

;R4  memory[R7+12]

;R7 is unchanged

LDMDB R7!, {R0, R2-R4} ;R0  memory[R7-16]

;R2  memory[R7-12]

;R3  memory[R7-8]

;R4  memory[R7-4]

;R7  R7 - 16

For use in conjunction with stack addressing, four alternative names can be used for

the addressing modes. These names are based on the type of stack being used instead

of the addressing mode being used. This eliminates confusion in coding stack push and

pop operations, since the type of stack will be the same for both the LDM and STM

instructions. In ARM syntax, a full stack is one where the stack pointer points to the last

used (full) location. An empty stack is one where the stack pointer points to the next

available (empty) stack location. As well, a stack can grow through increasing memory

addresses (ascending), or downward through decreasing memory addresses

(descending). The table below summarizes the stack addressing modes.

 FA (full ascending) - post-decrement (DA) on pop

 FD (full descending) - post-increment (IA) on pop

 EA (empty ascending) - pre-decrement (DB) on pop

 ED (empty descending) - pre-increment on (IB) pop

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ARM ARM7TDMI Bedienungsanleitung Seite 22