[raspi-internals] Vector Instructions: Column Base Offset
- From: Herman Hermitage <hermanhermitage@xxxxxxxxxxx>
- To: "raspi-internals@xxxxxxxxxxxxx" <raspi-internals@xxxxxxxxxxxxx>
- Date: Mon, 1 Jul 2013 22:59:42 +1200
Something I've witnessed in a few filters that do accumulation or blurring
across a few frames.
Status register bits SR[5:4] act as a column base offset.
Instructions to manipulate CB:
0000 0000 0000 0110 cbclr Clear vector column base ie SR[5:4]
= 0
0000 0000 0000 0111 cbinc Increment vector column base ie
SR[5:4]++
0000 0000 0000 1000 cbchg Toggle vector column base ie SR[5] =
~SR[5]
0000 0000 0000 1001 cbdec Decrement vector column base ie
SR[5:4]--
Then in the 80 bit vector instructions, the extended vector flags f_d, f_a, f_b:
5:2 Scalar register r0-r14, if required; otherwise, all 1's (ie r15 is not
available)
1 If set, increment argument by 1 for each instruction repetition.
Y coordinate is incremented if argument is horizontal; X coordinate if
vertical.
0 If set use bits 5:4 of SR as a column offset for the vector reference.
ie. P(y,x) becomes P(y,(x+cb*16)&61).
So setting the least significant bit, adjusts the vector coordinates according
to the current value of CB (SR[5:4]).
eg.
cbclr
vmov H(0,0)+CB, #100
cbinc
vmov H(0,0)+CB, #101
cbinc
vmov H(0,0)+CB, #102
cbinc
vmov H(0,0)+CB, #103
would result in H(0,0) = 100, H(0,16) = 101, H(0,32) = 102, H(0,48) = 103
The offset always seems to be horizontal, irrespective of the orientation of
the vector.
The main purpose seems to be simplifying kernels that need to refer to a
previous buffer. It saves gymnastics with scalar register offsets.
HH.
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