<!-- SVP64 Butterfly DCT Instructions here described are based on -->

<!-- PLEASE NOTE THESE ARE UNAPPROVED AND DRAFT, NOT SUBMITTED TO OPF ISA WG -->

# [DRAFT] Integer Butterfly Multiply Add/Sub Round Shift for FFT/DCT

A-Form

* maddsubrs  RT,RA,RB,SH

Pseudo-code:

    n <- SH
    sum <- (RT[0] || RT) + (RA[0] || RA)
    diff <- (RT[0] || RT) - (RA[0] || RA)
    prod1 <- MULS(RB, sum)
    prod2 <- MULS(RB, diff)
    if n = 0 then
        prod1_lo <- prod1[XLEN+1:(XLEN*2)]
        prod2_lo <- prod2[XLEN+1:(XLEN*2)]
        RT <- prod1_lo
        RS <- prod2_lo
    else
        round <- [0]*(XLEN*2 + 1)
        round[XLEN*2 - n + 1] <- 1
        prod1 <- prod1 + round
        prod2 <- prod2 + round
        res1 <- prod1[XLEN - n + 1:XLEN*2 - n]
        res2 <- prod2[XLEN - n + 1:XLEN*2 - n]
        RT <- res1
        RS <- res2

Special Registers Altered:

    None