A method of controlling instruction execution in a processing flow in a processing environment, wherein said processing flow includes two sections, one referred to as mainline processing and the other which is referred to as test set processing, said method comprising: executing an instruction routine of an instruction stream in the mainline processing, the instruction routine comprising one or more instructions, wherein the mainline processing is streamlined processing in which processing proceeds from one instruction routine to another instruction routine wherein the only test performed between instruction execution is a single test that determines whether mainline processing is to continue or whether processing is to fall through test set processing; performing, in response to executing the instruction routine, the single test, wherein the single test includes a comparison of an instruction counter to a variable, which is selectively set to a value; and executing the another instruction routine (612, 606), and thus remaining in mainline processing, or falling through to test set processing and executing the test set (612,614, 616), in response to the performing the single test, wherein the executing the test set, includes performing one or more tests or one or more actions, and also includes, if needed, the test set determining a next test set to be invoked in response to the processing again falling through mainline processing, the selecting of the next test set including setting a test set pointer to point to the next test set; wherein the executing the test set comprises using a buffer to contain an instruction to be executed (702, 704), wherein said instruction to be executed is an instruction that requires special handling in that at least one aspect associated with the instruction can be modified if needed, wherein the using the buffer comprises copying the instruction to the buffer to enable modifications to the instruction prior to executing the instruction; wherein subsequent to initializing the buffer, the test set is further processed by performing the steps of: redirecting an instruction pointer to the buffer; saving a current value of the variable used in the single test; saving a current value of the test set pointer; replacing these values with a value of zero and a value indicating a recovery test set, respectively; executing the instruction in the buffer; performing recovery by selecting the recovery test set wherein the redirecting step, saving steps, and replacing steps are undone; and continuing processing at the next instruction. executing an instruction routine of an instruction stream in the mainline processing, the instruction routine comprising one or more instructions, wherein the mainline processing is streamlined processing in which processing proceeds from one instruction routine to another instruction routine wherein the only test performed between instruction execution is a single test that determines whether mainline processing is to continue or whether processing is to fall through test set processing; performing, in response to executing the instruction routine, the single test, wherein the single test includes a comparison of an instruction counter to a variable, which is selectively set to a value; and executing the another instruction routine (612, 606), and thus remaining in mainline processing, or falling through to test set processing and executing the test set (612,614, 616), in response to the performing the single test, wherein the executing the test set, includes performing one or more tests or one or more actions, and also includes, if needed, the test set determining a next test set to be invoked in response to the processing again falling through mainline processing, the selecting of the next test set including setting a test set pointer to point to the next test set; wherein the executing the test set comprises using a buffer to contain an instruction to be executed (702, 704), wherein said instruction to be executed is an instruction that requires special handling in that at least one aspect associated with the instruction can be modified if needed, wherein the using the buffer comprises copying the instruction to the buffer to enable modifications to the instruction prior to executing the instruction; wherein subsequent to initializing the buffer, the test set is further processed by performing the steps of: redirecting an instruction pointer to the buffer; saving a current value of the variable used in the single test; saving a current value of the test set pointer; replacing these values with a value of zero and a value indicating a recovery test set, respectively; executing the instruction in the buffer; performing recovery by selecting the recovery test set wherein the redirecting step, saving steps, and replacing steps are undone; and continuing processing at the next instruction.