In general, upper 4 bits of Machine Instruction are assigned to an OP(eration)
code and lower 12-bits the Operand which points to a location of the memory.
Here we look at "JMI" (Jump if G is minus) instruction in detail.
Machine Instruction:
1001
0001 0111 1110
Assembly Instruction:
JMI
17Eh
Description:
If the bit 15 of G register is set(1), then
jump to execute the instruction in memory
at the location supplied by Operand - 17Ehex.
The sequence of operations to be performed within Fetch Cycle:
Subcycle
HDL
Note T6 T7 PC<-IR[0..11] if G[15]=1 Load the lower 12-bits of IR to PC
if the highest bit of G register is set(1)