APPENDIX A MIPS III INSTRUCTION SET DETAILS
Preliminary Users Manual S15543EJ1V0UM 543
SDR Store Doubleword Right (1/3) SDR
base

SDR

1 0 1 1 0 1 rt offset

31 26 25 21 20 16 15 0
655 16
Format:
SDR rt, offset (base)
Description:
This instruction can be used with the SDL instruction to store the contents of a register into eight consecutive bytes
of memory, when the bytes cross a boundary between two doublewords. SDR stores the register into the
appropriate part of the low-order doubleword; SDL stores the register into the appropriate part of the low-order
doubleword of memory.
The SDR instruction adds its sign-extended 16-bit
offset
to the contents of general register
base
to form a virtual
address that may specify an arbitrary byte. It alters only the word in memory that contains that byte. From one to
eight bytes will be stored, depending on the starting byte specified.
Conceptually, it starts at the least-significant byte of the register and copies it to the specified byte in memory; then
it copies bytes from register to memory until it reaches the high-order byte of the word in memory. No address
error exceptions due to alignment are possible.
address 8
address 0
memory
15 14 13 12 11 10 9 8
76543210 A B C D E F G H
before
after
$24
register

SDR $24, 1 ($0)

address 8
address 0
15 14 13 12 11 10 9 8
B C D E F G H 0