The **Casio 801-MR** is a bulky desktop calculator from 1975. It measures 116mm x 154mm x 37.5mm and weighs 280g without batteries. The case is made of black and grey glossy plastic with a tilted display escutcheon. The display shows 8 blue vacuum fluorescent digits with an unusual “small zero” design where only the bottom half lights up for zero. This was likely to allow digits to remain readable if some segments failed.

Power comes from 4 AA batteries or an optional AC adapter. Battery life is 14 hours with carbon batteries or 28 hours with alkalines. The 801-MR has a 4-function calculator with square root, percentages, and a 4-function memory. Additional features include:

- Switchable accumulating/decrementing memory
- Switchable constant
- 16 digit display shift to show additional digits

The keys are large, pronounced, and hollow sounding with a long key travel. The construction consists of a keyboard assembly sitting loosely on the main circuit board inside the case.

Some logic highlights:

- Clear (C) clears the input, All Clear (AC) fully resets
- Overflow is suppressed, 9th digit ignored
- Overflow shows 16 digit result but is not recoverable
- Divide by zero gives zero result
- Constant (K) works for multiply/divide only
- Negative numbers shown with “-” in leftmost digit
- Automatic accumulating memory stores 8 digits
- Negative square roots give positive result
- Display shift key (>) shows least significant 8 digits
- Suffers from divide by negative zero bug
- Negative zero bug

Capacity:

- Entry/display: 8 digits
- Add/subtract: 15 digits (9 integer)
- Multiply/divide: 8 digits
- Product: 16 digits
- Quotient: 8 significant figures
- Square root: 7 digits
- Memory: 8 digits

The Casio 801-MR is an early complex desktop calculator with some unusual display and memory features, but logic limitations like many models of the time. The “small zero” display is a standout, as is the dual equals keys for accumulation/decrementing memory. While bulky, the tactile keys still operate well after almost 50 years.