Encoding:

Format:

WSBH rt, rs

Purpose:

Word Swap Bytes Within Halfwords

To swap the bytes within each halfword of GPR rs and store the value into GPR rt.

Description:

 GPR[rt] = SwapBytesWithinHalfwords(GPR[rs])

Within each halfword of the lower word of GPR rs the bytes are swapped, the result is sign-extended, and stored in

GPR rt.

Restrictions:

In implementations prior to Release 2 of the architecture, this instruction resulted in a Reserved Instruction exception.

If GPR rs does not contain a sign-extended 32-bit value (bits 63..31 equal), then the result of the operation is

UNPREDICTABLE.

Operation:

if NotWordValue(GPR[rt]) then
   UNPREDICTABLE
endif
GPR[rt] = sign_extend(GPR[rs]23..16 || GPR[rs]31..24 || GPR[rs]7..0 || GPR[rs]15..8)

Exceptions:

Reserved Instruction

Programming Notes:

The WSBH instruction can be used to convert halfword and word data of one endianness to another endianness. The endianness of a word value can be converted using the following sequence:

lw    t0, 0(a1)           /* Read word value */
wsbh  t0, t0              /* Convert endiannes of the halfwords */
rotr  t0, t0, 16          /* Swap the halfwords within the words */

Combined with SEH and SRA, two contiguous halfwords can be loaded from memory, have their endianness converted, and be sign-extended into two word values in four instructions. For example:

lw    t0, 0(a1)           /* Read two contiguous halfwords */
wsbh  t0, t0              /* Convert endiannes of the halfwords */
seh   t1, t0              /* t1 = lower halfword sign-extended to word */
sra   t0, t0, 16          /* t0 = upper halfword sign-extended to word */

Zero-extended words can be created by changing the SEH and SRA instructions to ANDI and SRL instructions, respectively.

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