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The MIX computer has an internal register, called the *location
counter*, which stores the address of the next instruction to be fetched
and executed by the virtual CPU. You cannot directly modify the contents
of this internal register with a load instruction: after fetching the
current instruction from memory, it is automatically increased in one
unit by the MIX. However, there is a set of instructions (which we call
jump instructions) which can alter the contents of the location counter
provided some condition is met. When this occurs, the value of the next
instruction address that would have been fetched in the absence of the
jump is stored in ‘`rJ`’ (except for `JSJ`

), and the location
counter is set to the value of ‘`M`’ (so that the next instruction is
fetched from this new address). Later on, you can return to the point
when the jump occurred reading the address stored in ‘`rJ`’.

The MIX computer provides the following jump instructions:
With these instructions you force a jump to the specified address. Use
‘`JSJ`’ if you do not care about the return address.

`JMP`

Unconditional jump. OPCODE = 39, MOD = 0.

`JSJ`

Unconditional jump, but rJ is not modified. OPCODE = 39, MOD = 1.

These instructions check the overflow toggle to decide whether to jump or not.

`JOV`

Jump if OV is set (and turn it off). OPCODE = 39, MOD = 2.

`JNOV`

Jump if OV is not set (and turn it off). OPCODE = 39, MOD = 3.

In the following instructions, the jump is conditioned to the contents of the comparison flag:

`JL`

Jump if

`[CM] = L`

. OPCODE = 39, MOD = 4.`JE`

Jump if

`[CM] = E`

. OPCODE = 39, MOD = 5.`JG`

Jump if

`[CM] = G`

. OPCODE = 39, MOD = 6.`JGE`

Jump if

`[CM]`

does not equal`L`

. OPCODE = 39, MOD = 7.`JNE`

Jump if

`[CM]`

does not equal`E`

. OPCODE = 39, MOD = 8.`JLE`

Jump if

`[CM]`

does not equal`G`

. OPCODE = 39, MOD = 9.

You can also jump conditioned to the value stored in the MIX registers, using the following instructions:

`JAN`

`JAZ`

`JAP`

`JANN`

`JANZ`

`JANP`

Jump if the content of rA is, respectively, negative, zero, positive, non-negative, non-zero or non-positive. OPCODE = 40, MOD = 0, 1, 2, 3, 4, 5.

`JXN`

`JXZ`

`JXP`

`JXNN`

`JXNZ`

`JXNP`

Jump if the content of rX is, respectively, negative, zero, positive, non-negative, non-zero or non-positive. OPCODE = 47, MOD = 0, 1, 2, 3, 4, 5.

`JiN`

`JiZ`

`JiP`

`JiNN`

`JiNZ`

`JiNP`

Jump if the content of rIi is, respectively, negative, zero, positive, non-negative, non-zero or non-positive. OPCODE = 40 + i, MOD = 0, 1, 2, 3, 4, 5.