ARM is a load-store architecture:
- Data must be loaded into registers from memory before it can be operated upon.
- No instructions directly manipulate memory values.
Prior to ARMv4, ARM had no native support for loading halfwords and signed bytes.
unsignedby default on ARM.
Memory Instructions Throughout the Ages
Not all of the memory access instructions available in the current ARM ISA were present in the original ARM. Newer instructions, such as those for processing half-words, have had to be squeezed into later architectures. Limits on the amount of instruction space available meant that they could not be made as flexible as the original instructions.
||load unsigned 8-bit|
||store signed or unsigned 8-bit|
||load signed or unsigned 32-bit|
||store signed or unsigned 32-bit|
||load signed 8-bit|
||load unsigned 16-bit|
||load signed 16-bit|
||store signed or unsigned 16-bit|
||load signed or unsigned 64-bit|
||store signed or unsigned 64-bit|
You may get better performance by using
LDR to process packed pairs of half-words instead of
LDRH, for example.
ARMv4 introduced signed and halfword memory access types.
ARMv5 introduced double-word accesses.
STRDrequire addresses aligned to an 8-byte boundary.
ARM coding guru Robin Watts says: > “Using the newer types, such as half-word/short, is intrinsically less efficient on ARMs. Array indexing of shorts costs one more cycle than ints.”