Editing National Semiconductor SC/MP (section)

===ALU and main registers===
The [[arithmetic logic unit]] (ALU) was [[Accumulator (computing)|accumulator]]-based, with a single 8-bit accumulator, AC. A second 8-bit register, "Extension", or E, could be used as a backup for AC. E could not be accessed directly by most instructions; one could not load data from memory directly into E for instance. Instead, the value in AC could be read from E, or AC and E could be swapped. E can also be used as the source value for the seven logical/arithmetic operations supported by the ALU. If E were not used for other operations, it could also be used in a fashion similar to the mirror registers seen in designs like the [[Zilog Z80]], with E acting as a quick way to store the value of AC during an [[interrupt]].{{sfn|Handbook|1977|p=1.6}}

A major purpose of E was to act as a buffer for the built-in [[Serial communication|serial I/O]] system.{{sfn|Osborne|Kane|1981|p=3.3}} This was driven by the SIO instruction, which [[Bitwise operation#Bit shifts|bit shifted]] the least significant bit in E to a [[Flip-flop (electronics)|latch]] which was connected to the SOUT pin on the processor. Repeated calls to SIO thus presented bits of the 8-bit E value one at a time to the output, or reversed this operation for reading from the SIN pin.{{sfn|Handbook|1977|p=1.2, 1.10}} Once a complete 8-bit value had been read or written, the value could be sent to memory by exchanging the value with A and then loading or storing A to memory.{{sfn|Handbook|1977|p=1.6}} E could also be used as the source for logical operations on AC, so for instance, one could read a status byte in from the serial port and then AND that value with a mask in AC, to check for various status bits read from an external device.{{sfn|Handbook|1977|p=1.13}}

The separate Status Register was 8-bits. The three least-significant-bits, 0 through 2, were connected directly to pins, providing an easy way to check external device's status.{{sfn|Handbook|1977|p=1.6}} A further two pins, Sense A and B, were connected to bits 4 and 5. Sense A was normally used to service interrupts (see details below). Interrupts could be turned off by setting bit 3, Interrupt Enable (IE) to 0 with the Disable Interrupt instruction, DINT, and enabled with IEN, or by moving the value to and from A and manipulating it there. Bits 6 and 7 were conventional overflow and carry flags, as seen in most processors.{{sfn|Handbook|1977|p=1.3}}