I/O instructions. All told, 765 different types of instructions are used in the system.

d The instruction format (Fig. 3) makes use of both half and full words; half words accommodate indexing and floating-point instructions (for optimum performance these two sets of instructions use a rigid format), and full-word formats are used by the variable-field-length instructions. Notice that the latter specifies the operand field by the address of its left-most bit, the length of the field, and the byte¹ size, as well as the starting point (offset) of the implied operand (accumulator). Both halves of the word are independently indexable.

e A general monitoring device used for important status triggers is called the Interrupt [Brooks, 1957] System. This system monitors the flip-flops which reflect internal malfunctions, result significance (exponent range, mantissa zero, overflow, underflow), program errors (illegal instruction, protected memory area), and input/output conditions (unit not ready, etc.). The status of these flip-flops can cause a break in the normal progression of the stored program for fix-up purposes. Their status is automatically interrogated at all times.