JUN

Name

Jump Unconditionally

Function

Jump to any address within the memory space.

Syntax

JUN (address)

Assembled

Binary

0010RAAAA AAAAAAAA

Decimal

64, then incrementing by 1 until 79 (1st word)

Hexadecimal

0x40, then incrementing by 2 until 0x4F (1st word)

Symbolic

 ../../_images/jun-sym.png

Execution

2 words 16-bit code and an execution time of 21.6 \({\mu}\) sec

Side-effects

Not Applicable

Implemented

jun

Detailed Description

The 8 bits held in the register pair specified by RP are loaded into the lower 8 bits of the program counter. The highest 4 bits of the program counter are unchanged. Therefore program execution continues at this address on the same page of memory in which the JIN instruction is loaded.

The carry bit, nor the contents of the register pair are not affected.

The disassembly of the instruction below shows how the register pair is represented in the opcode.

../../_images/jun.png

This instruction and the JMS instruction , use a 12-bit address, and can reference any memory location. Their operation is not influenced by their position within a page of memory, whereas some other instructions are.

Therefore, only a JUN or JMS instruction should be used to transfer control from one page of memory to another.

Example program snippet for illustration

Arbitrary Memory
Address   (Hex)

  0x360               jun LRG
  0x362         AD,   add 1



  0x370         LAC,  ldm 3
  0x371               jun AD



  0x3EO         LRG,  fim 0p, 4
  0x3E2               jun LAC
                      end

Normally, program instructions are executed sequentially.

A 12-bit register called the program counter holds the address of the instruction to be executed. The JUN instruction replaces the program counter contents, causing program execution to continue at that address.

Thus the execution sequence of the above example is as follows:

The jun instruction at 0x360 replaces the contents of the program counter with 0x3EO. The next instruction executed is the fim at location LRG which loads register 0 with the value 0, and register 1 with the value 4.

The jun at 0x3E2 is then executed. The program counter is set to 0x370, and the ldm at this address loads the accumulator with the value 3.

The jun at 0x371 sets the program counter to 0x362, where the add instruction adds the contents of register 1 plus the carry bit to the accumulator.

From here, normal program execution continues at location 0x363.