Instruction Set
IBM 650 instructions consisted of a two-digit op code, a four-digit data address and the four-digit address of the next instruction. The sign was ignored. The 650 had a 20-digit accumulator, divided into 10-digit upper and lower sections with a common sign. Data read from the drum went through a 10-digit distributer. Instructions went to a program register. Arithmetic was performed by a one-digit adder. Additional instructions were provided for options, such as floating point, core storage, index registers and additional I/O devices. The base machine had 42 op codes. With all options installed, there were 97 op codes.
| 70 | RD | Read |
| 71 | PCH | Punch |
| 69 | LD | Load distributor |
| 24 | STD | Store distributor |
| 10 | AU | Add to upper |
| 15 | AL | Add to lower |
| 11 | SU | Subtract from upper |
| 16 | SL | Subtract from lower |
| 60 | RAU | Reset (entire accumulator) and add into upper |
| 65 | RAL | Reset and add into lower |
| 61 | RSU | Reset and subtract into upper |
| 66 | RSL | Reset and subtract into lower |
| 20 | STL | Store lower into memory |
| 21 | STU | Store upper into memory * |
| 22 | STDA | Store lower data address |
| 23 | STIA | Store lower instruction address |
| 17 | AABL | Add absolute to lower |
| 67 | RAABL | Reset and add absolute to lower |
| 18 | SABL | Subtract absolute from lower |
| 68 | RSABL | Reset and subtract absolute into lower |
| 19 | MULT | Multiply |
| 14 | DIV | Divide |
| 64 | DIVRU | Divide and reset upper |
| 44 | BRNZU | Branch on non-zero in upper |
| 45 | BRNZ | Branch on (accumulator) non-zero |
| 46 | BRMIN | Branch on minus |
| 47 | BROV | Branch on overflow |
| 90-99 | BRD | Branch on 8 in distributor positions 1-10 ** |
| 30 | SRT | Shift (accumulator) right |
| 31 | SRD | Shift and round |
| 35 | SLT | Shift left |
| 36 | SCT | Shift left and count *** |
| 84 | TLU | Table lookup |
| 00 | No-Op | No operation |
| 01 | Stop | Stop if console switch is set to stop, otherwise no-op |
Notes:
- * Value stored takes sign of accumulator, except after a divide operation; then sign of remainder is stored.
- ** Used to allow 533 control panel to signal CPU.
- *** Counts high-order zeros in upper accumulator
Read more about this topic: IBM 650
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