Jump Instructions on the CDC 160

This document is one section of the CDC 160 Programmer's Reference Manual.

Index

Relative Jump Instruction Format

            11 10 09 08 07 06 05 04 03 02 01 00
            ___________________________________
           |  |  |  |  |  |  |  |  |  |  |  |  |
           |__|__|__|__|__|__|__|__|__|__|__|__|
           |        |f/|     |                 |
           |        |/b|     |                 |
           |                 |                 |
           |     Opcode      |     Address     |
           |                 |                 |
           |     6-bit F     |     6-bit E     |
This group of instructions allows program counter relative jumps that are conditional on the value in the accumulator A. Bit 8, the f/b bit, determines whether the jump is forward (0) or backward (1). The destination address computation is as follows: 
    if f/b = 0
      then dst = P + E
      else dst = P - E
In all cases, jumping loads dst into PC; if the conditional jump fails, PC is merely incremented by 1, as usual for sequential execution.

110x00 - Zero Jump

Jump if all bits of A are zero.

110x01 - Non-Zero Jump

Jump if any bit of A is nonzero.

110x10 - Positive Jump

Jump if A is positive.

110x11 - Negative Jump

Jump if A is negative.

Indirect Jump Instruction Format

            11 10 09 08 07 06 05 04 03 02 01 00
            ___________________________________
           |  |  |  |  |  |  |  |  |  |  |  |  |
           |__|__|__|__|__|__|__|__|__|__|__|__|
           |                 |                 |
           |     Opcode      |     Address     |
           |                 |                 |
           |     6-bit F     |     6-bit E     |
This group of instructions allows indirect jumps to any location in memory.

111000 - Jump Indirect

Use E to address one of the first 64 memory locations, and load the contents of that location into P.

111001 - Jump Forward Indirect

Use forward relative addressing to fetch the destination address.

Function Calls

The CDC 160 has no instruction to both transfer control to a function and save the return address. Therefore, the programmer must create this effect using an instruction sequence. The most common way to do this on early CDC machines follows the following model, coded assuming the hardware upgrades released with Serial Number 37 and up (and probably retrofitted into most of the other 36 machines): 

x  : 010010000000 - Load Forward Relative
x+1:     x+6      - (return address)
x+2: 100001000000 - Store Indirect
x+3:      p       - (destination return link)
x+4: 111001000000 - Jump Forward Indirect
x+5:     p+1      - (destination address)
Typical receiving and return sequences for the above calling sequence might look like: 
p-1: 111001000000 - Jump Forward Relative
p  :      -       - (return address)
p+1:     ...      - Entry Point
         ...
         ...      - (function body)
         ...
p+n: 111001000000 - Jump Forward Indirect
   :     p-1      - (address of return jump)
For short functions where the sign or zero/nonzero status of the accumulator is known, the jump to the return jump at location p+n can be coded using a relative jump for efficiency.