Encoding:

Format:

LWR rt, offset(base)

Purpose:

Load Word Right

To load the least-significant part of a word from an unaligned memory address as a signed value

Description:

 GPR[rt] = GPR[rt] MERGE memory[GPR[base] + offset] 

The 16-bit signed offset is added to the contents of GPR base to form an effective address (EffAddr). EffAddr is the address of the least-significant of 4 consecutive bytes forming a word (W) in memory starting at an arbitrary byte boundary.

A part of W (the least-significant 1 to 4 bytes) is in the aligned word containing EffAddr. This part of W is loaded into the least-significant (right) part of the word in GPR rt. The remaining most-significant part of the word in GPR rt is unchanged.

If GPR rt is a 64-bit register, the destination word is the low-order word of the register. The loaded value is treated as a signed value; if the word sign bit (bit 31) is loaded (that is, when all 4 bytes are loaded), then the new sign bit value is copied into bits 63..32. If bit 31 is not loaded, the value of bits 63..32 is implementation dependent; the value is either unchanged or a copy of the current value of bit 31.

Executing both LWR and LWL, in either order, delivers a sign-extended word value in the destination register.

Restrictions:

None

Availability and Compatibility:

Release 6 removes the load/store-left/right family of instructions, and requires the system to support misaligned memory accesses.

Operation:

vAddr = sign_extend(offset) + GPR[base]
(pAddr, CCA) = AddressTranslation (vAddr, DATA, LOAD)
pAddr = pAddrPSIZE-1..3 || (pAddr2..0 xor ReverseEndian3)
if BigEndianMem = 0 then
   pAddr = pAddrPSIZE-1..3 || 03
endif
byte = vAddr1..0 xor BigEndianCPU2
word = vAddr2 xor BigEndianCPU
memdoubleword = LoadMemory (CCA, byte, pAddr, vAddr, DATA)
temp = GPR[rt]31..32-8*byte || memdoubleword31+32*word..32*word+8*byte
if byte = 4 then
   utemp = (temp31)32     /* loaded bit 31, must sign extend */
else
             /* one of the following two behaviors: */
   utemp = GPR[rt]63..32   /* leave what was there alone */ 
   utemp = (GPR[rt]31)32  /* sign-extend bit 31 */
endif
GPR[rt] = utemp || temp

Exceptions:

TLB Refill, TLB Invalid, Bus Error, Address Error, Watch

Programming Notes:

The architecture provides no direct support for treating unaligned words as unsigned values, that is, zeroing bits

63..32 of the destination register when bit 31 is loaded.

Historical Information:

In the MIPS I architecture, the LWL and LWR instructions were exceptions to the load-delay scheduling restriction.

A LWL or LWR instruction which was immediately followed by another LWL or LWR instruction, and used the same destination register would correctly merge the 1 to 4 loaded bytes with the data loaded by the previous instruction. All such restrictions were removed from the architecture in MIPS II.