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Architecture
188
SWL Store Word Left SWL
31 26 25 21 20 16 15 0
SWL
101010
base rt offset
6 5 5 16
Format :
SWL rt, offset(base)
Description :
Used together with SWR to store the contents of a register into four consecutive bytes of memory
when the bytes cross a word boundary. SWL stores the left part of the register into the appropriate
part of the high-order word in memory; SWR stores the right part of the register into the appropriate
part of the low-order word in memory.
This instruction generates a 32-bit effective address that can point to any byte by sign-extending the
16-bit offset and adding it to the contents of general-purpose register base. Only the one word in
memory containing the designated starting byte is modified. Depending on the starting byte, from
one to four bytes are stored.
The concept is illustrated below. This instruction (SWL) starts from the high-order (left-most) byte
of the register and stores it into the designated memory byte; it then continues storing bytes from
register to memory, proceeding toward the low-order byte of the register and the low-order byte of
the memory word, until it reaches the low-order byte of the memory word.
No Address Error instruction is raised due to misalignment.
Memory
(Big endian)
Before
storing
SWL $24,1($0)
After
storing
Address 4 4 5 6 7
Address 0 0 A B C
Address 4 4 5 6 7
Address 0 0 1 2 3
Register
A B C D $24