[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