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In some instruction descriptions there are calls to SignedSatQ (directly or indirectly).
The pseudocode for SignedSatQ:
(bits(N), boolean) SignedSatQ(integer i, integer N) if i > 2^(N-1) - 1 then result = 2^(N-1) - 1; saturated = TRUE; elsif i < -(2^(N-1)) then result = -(2^(N-1)); saturated = TRUE; else result = i; saturated = FALSE; return (result<N-1:0>, saturated);
(bits(N), boolean) SignedSatQ(integer i, integer N)
if i > 2^(N-1) - 1 then
result = 2^(N-1) - 1; saturated = TRUE;
elsif i < -(2^(N-1)) then
result = -(2^(N-1)); saturated = TRUE;
else
result = i; saturated = FALSE;
return (result<N-1:0>, saturated);
What's the idea with 'result<N-1:0>' in the return (shifting the result one bit left)?
In pseudocode integers and bitstrings are different things. Integers are arbitrary precision and bitstrings are, well, strings of bits...
In this function, "result" is an integer. To convert an integer to a bitstring, you can apply the bit splice operation. Hence "result<N-1:0>" is just converting the integer to an N-bit string (i.e. extracting bits N-1 to 0). It's not shifting the result.
Thanks. Only now I notice the difference - funny.
I was confusing this: "(-, shift_n) = DecodeImmShift(‘10’, imm3:imm2);"
and this (in angle brackets): "result<N-1:0>"
Range versus catenation.