Pseudo-Random Binary Sequence Generator; Multiplication By Constant Using The Barrel Shifter - Samsung S3C2501X User Manual

S3C2501X
5. Overflow in unsigned multiply accumulate with a 64 bit result
UMULL
ADDS
ADC
BCS
6. Overflow in signed multiply accumulate with a 64 bit result
SMULL
ADDS
ADC
BVS
Overflow checking is not applicable to unsigned and signed multiplies with a 64-bit result, since overflow
does not occur in such calculations.

3.18.2 PSEUDO-RANDOM BINARY SEQUENCE GENERATOR

It is often necessary to generate (pseudo-) random numbers and the most efficient algorithms are based on shift
generators with exclusive-OR feedback rather like a cyclic redundancy check generator. Unfortunately the
sequence of a 32 bit generator needs more than one feedback tap to be maximal length (i.e. 2^32-1 cycles
before repetition), so this example uses a 33 bit register with taps at bits 33 and 20. The basic algorithm is
newbit: = bit 33 eor bit 20, shift left the 33 bit number and put in newbit at the bottom; this operation is performed
for all the newbits needed (i.e. 32 bits). The entire operation can be done in 5 S cycles:
TST
MOVS
ADC
EOR
EOR

3.18.3 MULTIPLICATION BY CONSTANT USING THE BARREL SHIFTER

Multiplication by 2^n (1,2,4,8,16,32..)
MOV
Multiplication by 2^n+1 (3,5,9,17..)
ADD
Multiplication by 2^n-1 (3,7,15..)
RSB
Rl,Rh,Rm,Rn
Rl,Rl,Ra1
Rh,Rh,Ra2
overflow
Rl,Rh,Rm,Rn
Rl,Rl,Ra1
Rh,Rh,Ra2
overflow
Rb,Rb,LSR#1
Rc,Ra,RRX
Rb,Rb,Rb
Rc,Rc,Ra,LSL#12
Ra,Rc,Rc,LSR#20
Ra, Rb, LSL #n
Ra,Ra,Ra,LSL #n
Ra,Ra,Ra,LSL #n
; 3 to 6 cycles
; Lower accumulate
; Upper accumulate
; 1 cycle and 2 registers
; 3 to 6 cycles
; Lower accumulate
; Upper accumulate
; 1 cycle and 2 registers
NOTE
; Enter with seed in Ra (32 bits),
; Rb (1 bit in Rb lsb), uses Rc.
; Top bit into carry
; 33 bit rotate right
; Carry into lsb of Rb
; (involved!)
; (similarly involved!) new seed in Ra, Rb as before
INSTRUCTION SET
3-61