IBM 650 - Instruction Set

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

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