OpCodes 0x00 ~ 0x0F

0x00: HLT

Halts the CPU6

The HLT instruction will check the current interrupt level. If the interrupt level is 15, then the CPU6 enters the Halt state which can be exited with a backplane interrupt or DMA interrupt, or by toggling the R or I dip switches. If the interrupt level is not 15, then HLT will trigger an abort trap with value 1.

Example:

      XFR=     X'00FF',Z    ; Load Z reg. with a value of FF
      CLR      X            ; Clear X register to zero
TOP   INR      X            ; Increment X reg. by one
      XFR      X,Y          ; Transfer counter into Y
      SUB      Z,Y          ; Subtracts Z-Y
      BNZ      TOP          ; Branch if Not Zero to TOP
      HLT                   ; If Zero, restart whole program

This is a simple loop that initially sets Z to a value of 0x00FF and clears the X register to 0. It then copies X to Y and subtracts Y from. It then tests if the result is 0, and if it is not zero (ie. Z is not equal to Y), it jumps back to TOP, increments X and tests again. If the result is zero, it skips to HLT which will Halt the CPU. This illuminate the HALT LED on the front panel and the system must be restarted from there.

0x01: NOP

No operation

NOP does not have any microcode, the initial instruction decoding provides the microcode address for the start of the instruction loop.

Example: NOP

0x02: SF

Set the fault flag

Other flags are unaffected. MORE EXPLANATION.

Example:

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0x03: RF

Reset the Fault flag to zero

Other flags are unaffected. MORE EXPLANATION.

Example:

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0x04: EI

Enable Interrupt

MORE EXPLANATION.

Example:

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0x05: DI

Disable Interrupt

MORE EXPLANATION.

Example:

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0x06: SL

Set Link/carry

MORE EXPLANATION.

Example:

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0x07: RL

Reset Link/carry

MORE EXPLANATION.

Example:

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0x08: CL

Complement/invert link/carry

MORE EXPLANATION.

Example:

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0x09: RSR

Return from SubRoutine

When a Jump SubRoutine instruction is run, the X register is pushed to the stack to save its current value, the PC is copied to X, and the effective address (EA) is then copied to the PC. When the program hits the Return from SubRoutine instruction, X is copied into the PC and then X is popped from the stack to restore it's original value.

There is a lot more going on behind the scenes, but from a programmer's perspective, this can be thought of as mostly standard subroutine control behavior.

Example:

      CLR      X,1          ; Clears X register and initializes with a 1
LOOP  XFR      X,Y          ; Transfer X register into Y
      JSR/     DELY         ; Jump to DELY subroutine
      INR      X            ; Increment X value
      JMP      LOOP         ; Jump to LOOP label
DELY  DLY                   ; 4.55ms delay
      DCR      Y            ; Decrement Y value
      BNZ      DELY         ; Branch if Not Zero to DELY
      RSR                   ; Return from Subroutine

This is a short loop that initially sets the Y register to 0 and then loops forever. Each iteration the Y register increments, meaning the DELY subroutine loops and delays for longer and longer each time. The RSR instruction at the end tells the subroutine to return to the instruction after JSR that sent it to the subroutine initially and to continue along.

0x0A: RI

Return from interrupt

MORE EXPLANATION.

Example:

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0x0B: Illegal Operation

Illegal instructions will trigger an abort trap. The instruction 0B has a different initial instruction decoding from the rest of the illegal instructions. On the EE200 this is an RIM instruction. It's possible that this instruction was never used by Centurion and was replaced by a user mode instruction on the CPU5. Possibly it was removed from the CPU6 as it was replaced by another instruction.

0x0C: SYN

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MORE EXPLANATION.

Example:

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0x0D: PCX

Transfer PC to X

Example:

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0x0E: DLY

4.55 ms delay

Example:

DELY  DLY                   ; 4.55ms delay
      DCR      Y            ; Decrement Y value
      BNZ      DELY         ; Branch if Not Zero to DELY
      RSR                   ; Return from Subroutine

This is short delay subroutine that counts down the Y value and loops back to the label DELY until the Y value reaches 0, at which point it returns from the subroutine. Each time it loops back to the label, it executes the DLY instruction, which cause the CPU to delay for 4.55ms. If the Y value is 0x0F, the subroutine will loop 16 times, resulting in a delay of 72.8ms.

0x0F: RSV

Return from SVC

Example:

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