A.1 Floating-Point Round to Single-Precision Model

The following describes algorithmically the operation of the Floating Round to Single-Precision instruction.

def FRSP(FRB, FPSCR):
    if ((FRB)[1:11] <u 897) & ((FRB)[1:63] >u 0) then
        if FPSCR['UE'] = 0 then
            return FRSP_Disabled_Exponent_Underflow(FRB, FPSCR)
        if FPSCR['UE'] = 1 then
            return FRSP_Enabled_Exponent_Underflow(FRB, FPSCR)

    if ((FRB)[1:11] >u 1150) & ((FRB)[1:11] <u 2047) then
        if FPSCR['OE'] = 0 then
            return FRSP_Disabled_Exponent_Overflow(FRB, FPSCR)
        if FPSCR['OE'] = 1 then
            return FRSP_Enabled_Exponent_Overflow(FRB, FPSCR)

    if ((FRB)[1:11] >u 896) & ((FRB)[1:11] <u 1151) then
        return FRSP_Normal_Operand(FRB, FPSCR)

    if (FRB)[1:63] = 0 then
        return FRSP_Zero_Operand(FRB, FPSCR)

    if (FRB)[1:11] = 2047 then
        if (FRB)[12:63] = 0 then
            return FRSP_Infinity_Operand(FRB, FPSCR)
        if (FRB)[12] = 1 then
            return FRSP_QNaN_Operand(FRB, FPSCR)
        if ((FRB)[12] = 0) & ((FRB)[13:63] >u 0) then
            return FRSP_SNaN_Operand(FRB, FPSCR)

def FRSP_Disabled_Exponent_Underflow(FRB, FPSCR):
    sign <- (FRB)[0]
    frac[0:52] <- 0
    exp <- 0
    if (FRB)[1:11] = 0 then
        exp <- -1022
        frac[0:52] <- 0b0 || (FRB)[12:63]
    if (FRB)[1:11] >u 0 then
        exp <- (FRB)[1:11] - 1023
        frac[0:52] <- 0b1 || (FRB)[12:63]

    # Denormalize operand:
    G <- 0b0
    R <- 0b0
    X <- 0b0
    do while exp < -126
        exp <- exp + 1
        X <- X | R
        R <- G
        G <- frac[52]
        frac[0:52] <- 0b0 || frac[0:51]

    FPSCR['UX'] <- (frac[24:52] || G || R || X) >u 0
    exp, frac, FPSCR <- Round_Single(sign, exp, frac[0:52], G, R, X, FPSCR)
    FPSCR['XX'] <- FPSCR['XX'] | FPSCR['FI']
    FRT <- [0b0] * 64
    if frac[0:52] = 0 then
        FRT[0] <- sign
        FRT[1:63] <- 0
        if sign = 0 then FPSCR['FPRF'] <- '+ zero'
        if sign = 1 then FPSCR['FPRF'] <- '- zero'
    if frac[0:52] >u 0 then
        if frac[0] = 1 then
            if sign = 0 then FPSCR['FPRF'] <- '+ normal number'
            if sign = 1 then FPSCR['FPRF'] <- '- normal number'
        if frac[0] = 0 then
            if sign = 0 then FPSCR['FPRF'] <- '+ denormalized number'
            if sign = 1 then FPSCR['FPRF'] <- '- denormalized number'

        # Normalize operand:
        do while frac[0] = 0
            exp <- exp-1
            frac[0:52] <- frac[1:52] || 0b0

        FRT[0] <- sign
        FRT[1:11] <- exp + 1023
        FRT[12:63] <- frac[1:52]
    return FRT, FPSCR

def FRSP_Enabled_Exponent_Underflow(FRB, FPSCR):
    FPSCR['UX'] <- 1
    sign <- (FRB)[0]
    frac <- [0b0] * 53
    exp <- 0
    if (FRB)[1:11] = 0 then
        exp <- -1022
        frac[0:52] <- 0b0 || (FRB)[12:63]
    if (FRB)[1:11] >u 0 then
        exp <- (FRB)[1:11] - 1023
        frac[0:52] <- 0b1 || (FRB)[12:63]

    # Normalize operand:
    do while frac[0] = 0
        exp <- exp - 1
        frac[0:52] <- frac[1:52] || 0b0

    exp, frac, FPSCR <- Round_Single(sign, exp, frac[0:52], 0b0, 0b0, 0b0, FPSCR)
    FPSCR['XX'] <- FPSCR['XX'] | FPSCR['FI']
    exp <- exp + 192
    FRT <- [0b0] * 64
    FRT[0] <- sign
    FRT[1:11] <- exp + 1023
    FRT[12:63] <- frac[1:52]
    if sign = 0 then FPSCR['FPRF'] <- '+ normal number'
    if sign = 1 then FPSCR['FPRF'] <- '- normal number'
    return FRT, FPSCR

def FRSP_Disabled_Exponent_Overflow(FRB, FPSCR):
    FPSCR['OX'] <- 1
    FRT <- [0b0] * 64
    if FPSCR['RN'] = 0b00 then             # Round to Nearest
        if (FRB)[0] = 0 then FRT <- 0x7FF0_0000_0000_0000
        if (FRB)[0] = 1 then FRT <- 0xFFF0_0000_0000_0000
        if (FRB)[0] = 0 then FPSCR['FPRF'] <- '+ infinity'
        if (FRB)[0] = 1 then FPSCR['FPRF'] <- '- infinity'
    if FPSCR['RN'] = 0b01 then             # Round toward Zero
        if (FRB)[0] = 0 then FRT <- 0x47EF_FFFF_E000_0000
        if (FRB)[0] = 1 then FRT <- 0xC7EF_FFFF_E000_0000
        if (FRB)[0] = 0 then FPSCR['FPRF'] <- '+ normal number'
        if (FRB)[0] = 1 then FPSCR['FPRF'] <- '- normal number'
    if FPSCR['RN'] = 0b10 then             # Round toward +Infinity
        if (FRB)[0] = 0 then FRT <- 0x7FF0_0000_0000_0000
        if (FRB)[0] = 1 then FRT <- 0xC7EF_FFFF_E000_0000
        if (FRB)[0] = 0 then FPSCR['FPRF'] <- '+ infinity'
        if (FRB)[0] = 1 then FPSCR['FPRF'] <- '- normal number'
    if FPSCR['RN'] = 0b11 then             # Round toward -Infinity
        if (FRB)[0] = 0 then FRT <- 0x47EF_FFFF_E000_0000
        if (FRB)[0] = 1 then FRT <- 0xFFF0_0000_0000_0000
        if (FRB)[0] = 0 then FPSCR['FPRF'] <- '+ normal number'
        if (FRB)[0] = 1 then FPSCR['FPRF'] <- '- infinity'
    FPSCR['FR'] <- undefined(0) # FIXME: figure out what values POWER9 uses
    FPSCR['FI'] <- 1
    FPSCR['XX'] <- 1
    return FRT, FPSCR

def FRSP_Enabled_Exponent_Overflow(FRB, FPSCR):
    sign <- (FRB)[0]
    exp <- (FRB)[1:11] - 1023
    frac <- [0b0] * 53
    frac[0:52] <- 0b1 || (FRB)[12:63]
    exp, frac, FPSCR <- Round_Single(sign, exp, frac[0:52], 0b0, 0b0, 0b0, FPSCR)
    FPSCR['XX'] <- FPSCR['XX'] | FPSCR['FI']
    # Enabled Overflow:
    FPSCR['OX'] <- 1
    exp <- exp - 192
    FRT <- [0b0] * 64
    FRT[0] <- sign
    FRT[1:11] <- exp + 1023
    FRT[12:63] <- frac[1:52]
    if sign = 0 then FPSCR['FPRF'] <- '+ normal number'
    if sign = 1 then FPSCR['FPRF'] <- '- normal number'
    return FRT, FPSCR

def FRSP_Zero_Operand(FRB, FPSCR):
    FRT <- (FRB)
    if (FRB)[0] = 0 then FPSCR['FPRF'] <- '+ zero'
    if (FRB)[0] = 1 then FPSCR['FPRF'] <- '- zero'
    FPSCR['FRFI'] <- 0b00
    return FRT, FPSCR

def FRSP_Infinity_Operand(FRB, FPSCR):
    FRT <- (FRB)
    if (FRB)[0] = 0 then FPSCR['FPRF'] <- '+ infinity'
    if (FRB)[0] = 1 then FPSCR['FPRF'] <- '- infinity'
    FPSCR['FRFI'] <- 0b00
    return FRT, FPSCR

def FRSP_QNaN_Operand(FRB, FPSCR):
    FRT <- (FRB)[0:34] || [0b0] * 29
    FPSCR['FPRF'] <- 'QNaN'
    FPSCR['FR'] <- 0b0
    FPSCR['FI'] <- 0b0
    return FRT, FPSCR

def FRSP_SNaN_Operand(FRB, FPSCR):
    FPSCR['VXSNAN'] <- 1
    FRT <- [0b0] * 64
    if FPSCR['VE'] = 0 then
        FRT[0:11] <- (FRB)[0:11]
        FRT[12] <- 1
        FRT[13:63] <- (FRB)[13:34] || [0b0] * 29
        FPSCR['FPRF'] <- 'QNaN'
    FPSCR['FR'] <- 0b0
    FPSCR['FI'] <- 0b0
    return FRT, FPSCR

def FRSP_Normal_Operand(FRB, FPSCR):
    sign <- (FRB)[0]
    exp <- (FRB)[1:11] - 1023
    frac <- [0b0] * 53
    frac[0:52] <- 0b1 || (FRB)[12:63]
    exp, frac, FPSCR <- Round_Single(sign, exp, frac[0:52], 0b0, 0b0, 0b0, FPSCR)
    FPSCR['XX'] <- FPSCR['XX'] | FPSCR['FI']
    if (exp > 127) & (FPSCR['OE'] = 0) then
        return FRSP_Disabled_Exponent_Overflow(FRB, FPSCR)
    if (exp > 127) & (FPSCR['OE'] = 1) then
        return FRSP_Enabled_Overflow(FRB, FPSCR)
    FRT <- [0b0] * 64
    FRT[0] <- sign
    FRT[1:11] <- exp + 1023
    FRT[12:63] <- frac[1:52]
    if sign = 0 then FPSCR['FPRF'] <- '+ normal number'
    if sign = 1 then FPSCR['FPRF'] <- '- normal number'
    return FRT, FPSCR

def Round_Single(sign, exp, frac, G, R, X, FPSCR):
    inc <- 0
    lsb <- frac[23]
    gbit <- frac[24]
    rbit <- frac[25]
    xbit <- (frac[26:52]||G||R||X) != 0
    if FPSCR['RN'] = 0b00 then                       # Round to Nearest
        # comparisons ignore u bits
        if (lsb || gbit) = 0b11 then inc <- 1
        if (lsb || gbit || rbit) = 0b011 then inc <- 1
        if (lsb || gbit || xbit) = 0b011 then inc <- 1
    if FPSCR['RN'] = 0b10 then                       # Round toward + Infinity
        # comparisons ignore u bits
        if (sign || gbit) = 0b01 then inc <- 1
        if (sign || rbit) = 0b01 then inc <- 1
        if (sign || xbit) = 0b01 then inc <- 1
    if FPSCR['RN'] = 0b11 then                       # Round toward - Infinity
        # comparisons ignore u bits
        if (sign || gbit) = 0b11 then inc <- 1
        if (sign || rbit) = 0b11 then inc <- 1
        if (sign || xbit) = 0b11 then inc <- 1
    frac[0:23] <- frac[0:23] + inc
    if (inc = 1) & (frac[0:23] = 0) then
        frac[0:23] <- 0b1 || frac[0:22]
        exp <- exp + 1
    frac[24:52] <- [0b0] * 29
    FPSCR['FR'] <- inc
    FPSCR['FI'] <- gbit | rbit | xbit
    return exp, frac, FPSCR

def DOUBLE(WORD):
    exp <- [0] * 11
    frac <- [0] * 53
    sign <- 0b0
    FRT <- [0] * 64
    # Normalized Operand
    if (WORD[1:8] >u 0) & (WORD[1:8] <u 255) then
        FRT[0:1] <- WORD[0:1]
        FRT[2] <- ¬WORD[1]
        FRT[3] <- ¬WORD[1]
        FRT[4] <- ¬WORD[1]
        FRT[5:63] <- WORD[2:31] || [0]*29
    # Denormalized Operand
    if (WORD[1:8] = 0) & (WORD[9:31] != 0) then
        sign <- WORD[0]
        exp <- -126
        frac[0:52] <- 0b0 || WORD[9:31] || [0]*29
        #normalize the operand
        do while frac[0] = 0
            frac[0:52] <- frac[1:52] || 0b0
            exp <- exp - 1
        FRT[0] <- sign
        FRT[1:11] <- exp + 1023
        FRT[12:63] <- frac[1:52]
    # Zero / Infinity / NaN
    if (WORD[1:8] = 255) | (WORD[1:31] = 0) then
        FRT[0:1] <- WORD[0:1]
        FRT[2] <- WORD[1]
        FRT[3] <- WORD[1]
        FRT[4] <- WORD[1]
        FRT[5:63] <- WORD[2:31] || [0]*29
    return FRT