The two memory test instructions were: — increment the memory location, and skip the next instruction if the result is zero. — decrement the memory location, and skip the next instruction if the result is zero. As in the case of the load and store instructions, there was an indirect bit that would perform a single level of indirect addressing. These instructions were odd in that, on the Novas with magnetic core memory, the instruction was executed within the memory board itself. As was common at the time, the memory boards contained a "write-back" circuit to solve the destructive-read problem inherent to magnetic core memory. But the write-back mechanism also contained a mini arithmetic unit, which the processor used for several purposes. For the ISZ and DSZ instructions, the increment or decrement occurred between the memory location being read and the write-back; the CPU simply waited to be told if the result was zero or nonzero. These instructions were useful because they allowed a memory location to be used as a loop counter without tying up an accumulator, but they were slower than performing the equivalent arithmetic instructions.
The two memory test instructions were: — increment the memory location, and skip the next instruction if the result is zero. — decrement the memory location, and skip the next instruction if the result is zero. As in the case of the load and store instructions, there was an indirect bit that would perform a single level of indirect addressing. These instructions were odd in that, on the Novas with magnetic core memory, the instruction was executed within the memory board itself. As was common at the time, the memory boards contained a "write-back" circuit to solve the destructive-read problem in!herent to magnetic core memory. But the write-back mechanism also contained a mini arithmetic unit, which the processor used for several purposes. For the ISZ and DSZ i,nstructions, the increment or decrement occurred between the memory location being read and the write-back; the CPU simply waited to be told if the result was zero or nonzero. These instructions were useful because they allowed a memory location to be used as a loop counter without tying up an accumulator, but they were slower than performing the equivalent arithmetic instruc.tions.