  In this paper we deal with semistable sheaves which can be represented as the
cokernel of an injective (or as the kernel of a surjective) morphism
$E_1\otimes\CC^m\longrightarrow E_2\otimes\CC^n$ , where $E_1$ and $E_2$ are
exceptional bundles. To each such a sheaf we assign the linear map
$\CC^m\otimes\hom^*(E_1,E_2)\longrightarrow \CC^n$. We obtain sufficient
conditions on the topological invariants of the sheaves for the moduli space of
the sheaves to be isomorphic to the quotient of an open subset in $\PP{\cal
L}(\CC^m\otimes\hom^*(E_1,E_2),\CC^n)$ under the action of
SL$(\CC^m)\times$SL$(\CC^n)$.
