;Example 3 - 17. Scaling for BFP Based FFT Implementation ASM Listing for TMS320C55x
;****************************************************************
; scale_exp: Find maximum absolute value in array specified by
; address AR3 and length T0. Pre-scale the input to next
; radix-2 stage by 2^(exponent of the maximum absolute
; value - 1) for first two stages and by 2^(exponent of
; the maximum absolute value - 2) for all following stages
;****************************************************************
scale_exp:
PSH dbl(AC0) ; save registers to be modified(stack size += 12)
PSH dbl(AC1)
PSH dbl(AC2)
PSH dbl(AC3)
PSH AR3
PSH T0
PSH T1
; --------find maximum absolute value (real and imaginary)-------
MOV *sp(data_ptr + 12), AR3 ; load array size into AR3 (sp
; offset computed from
MOV *sp(data_sz + 12), T0 ; above pushed registers and the
; return address
SUB #2, T0
|| MOV dbl(*AR3+), AC0 ; AC0_H = x_Re[0], AC0_L = x_Im[1]
MOV T0, BRC0 ; BRC0 = NX - 2
|| MOV dbl(*AR3), AC1 ; AC1_H = x_Re[2], AC1_L = x_Im[3]
MOV dbl(*AR3+), AC2 ; AC2_H = x_Re[2], AC1_L = x_Im[3]
|| RPTBLOCAL find_max
MAXDIFF AC0, AC1, AC1, AC3 ; AC1_H/L contain max's
find_max:
MOV dbl(*AR3+), AC0 ; AC0_H = x_Re[n], AC0_L = x_Imn+1]
|| MINDIFF AC0, AC2, AC2, AC3 ; AC2_H/L contain min's
MOV HI(AC1), AR3 ; AR3 = even max value
MOV AC1, T0 ; T0 = odd max value
MAX AR3, T0
MOV HI(AC2), T1 ; T1 = odd min value
MOV AC2, AR3 ; AR3 = even min value
MIN AR3, T1
ABS T1
MAX T0, T1
MOV T1, HI(AC0) ; put maximum absolute value in high part of AC0
; -------scale array according to maximum asbolute value---------
EXP AC0, T1 ; determine exponent from maximum absolute value
MOV *sp(bit_buf + 12), AR3 ; AR3 contains expected bit growth
SUB AR3, T1 ; subtract 1 or 2 from the exponent to compensate
; for the maximum possible bit growth (factor of
; 2 or 4) in the next radix-2 stage
BCC no_scale, T1 == #0 ; branch if no scaling needed
ADD *sp(ret_exp + 12), T1, AC0 ; sum current with previous exp.
MOV *sp(data_sz + 12), T0 ; put nx in T0 for next loop
MOV AC0, *sp(ret_exp + 12) ; put exponent back on stack
|| SUB #1, T0
MOV *sp(data_ptr + 12), AR3
|| MOV T0, BRC0 ; initialize loop counter (BRC0 = nx - 1)
RPTBLOCAL scale
MOV dbl(*AR3), pair(LO(AC0))
MOV AC0
scale:
MOV AC1 no_scale:
POP T1 ; restore registers
POP T0
POP AR3
POP dbl(AC3)
POP dbl(AC2)
POP dbl(AC1)
POP dbl(AC0)
RET
