2.1.110. --split_ldm

This option instructs the compiler to split LDM and STM instructions into two or more LDM or STM instructions.

When --split_ldm is selected, the maximum number of register transfers for an LDM or STM instruction is limited to:

Where register transfers beyond these limits are required, multiple LDM or STM instructions are used.

Note

This option is deprecated and will be removed in the next release.

Usage

The --split_ldm option can be used to reduce interrupt latency on ARM systems that:

  • do not have a cache or a write buffer, for example, a cacheless ARM7TDMI

  • use zero-wait-state, 32-bit memory.

Note

Using --split_ldm increases code size and decreases performance slightly.

Restrictions

  • Inline assembler LDM and STM instructions are split by default when --split_ldm is used. However, the compiler might subsequently recombine the separate instructions into an LDM or STM.

  • VFP FLDM or FSTM instructions are not split when --split_ldm is used.

  • Some target hardware does not benefit from code built with --split_ldm. For example:

    • It has no significant benefit for cached systems, or for processors with a write buffer.

    • It has no benefit for systems with non zero-wait-state memory, or for systems with slow peripheral devices. Interrupt latency in such systems is determined by the number of cycles required for the slowest memory or peripheral access. Typically, this is much greater than the latency introduced by multiple register transfers.

See also

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