The FORTRAN level-3 BLAS call for this operation is

SUBROUTINE TRSM ( SIDE, UPLO, TRANS, DIAG, M, N, ALPHA, A, LDA, B, LDB )

Here is replaced by S, D, C, or Z
to indicate the datatype of the data (single, double, complex, double precision complex).
Parameter SIDE indicates whether multiplies B from
the left (SIDE = "Left") or right (SIDE = "Right").
Parameter UPLO indicates whether A is
lower (UPLO = "Lower") or upper (UPLO = "Upper") triangular.
Parameter TRANS indicates if A is to be transposed as part of
the operation (TRANS = "No transpose", "Transpose", or "Conjugate transpose").
Parameter DIAG indicates if the diagonal of
A is to be taken to equal the identity matrix
(DIAG = "Unit" and or )
or the values in the matrix
(DIAG = "Non unit" and A = U or A = L ).
Parameters M and N pass the dimensions of matrix B .
The dimension of A is determined by parameter SIDE.
Parameter ALPHA holds the scaling factor .
Parameters A and B equal the
addresses of where the matrices A and B start in
memory. The leading dimensions of the matrices are given in
LDA and LDB.