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Calculate Instruction Time

Estimate the execution time of a PDP-1 instruction.

Instruction Time Breakdown

PDP-1 Instruction Base Cycles

Instruction Type	Base Cycles (Typical)	Notes
Memory Reference (Direct)	2	Fetch + Execute
Memory Reference (Indirect)	2 + N	Fetch + Execute + N indirect cycles
IOT	1 + Wait	1 cycle + device wait time
Operate Group (Basic)	1	Single cycle operations
Operate Group (Shifts)	1 + M	1 base + M extra for M shifts
SKP	1 or 2	1 if no skip, 2 if skip taken
JMP / JSP	1	Jump or Jump to Subroutine

Typical base cycle counts for PDP-1 instructions. 1 cycle = Memory Cycle Time.

What is a PDP-1 Instruction Timing Calculator?

A PDP-1 Instruction Timing Calculator is a tool used to estimate the time it takes for the Digital Equipment Corporation (DEC) PDP-1 computer to execute a single machine instruction. The PDP-1, a seminal computer from the early 1960s, had instruction execution times that were directly related to its core memory cycle time and the type of instruction being performed. This calculator helps understand the performance characteristics of this historic machine.

It’s primarily used by computer history enthusiasts, students of computer architecture, and researchers studying early computing systems. By inputting the memory cycle time (typically 5 microseconds for the PDP-1), the instruction type, and other relevant parameters like indirect addressing levels or I/O wait times, the PDP-1 Instruction Timing Calculator provides an estimate of the execution time in microseconds.

A common misconception is that all instructions on the PDP-1 took the same amount of time. In reality, instruction timing varied based on memory access patterns (direct vs. indirect addressing), I/O operations, and the complexity of micro-operations within an instruction like multiple shifts. Our PDP-1 Instruction Timing Calculator accounts for these variations.

PDP-1 Instruction Timing Formula and Mathematical Explanation

The time taken to execute a PDP-1 instruction depends primarily on the number of memory cycles required. A memory cycle was the fundamental time unit, typically 5 microseconds (µs) for the PDP-1’s core memory.

The general formula is:

Total Time = (Base Cycles + Indirect Cycles + Extra Cycles) * Memory Cycle Time + I/O Wait Time

Where:

• Base Cycles: The minimum number of memory cycles an instruction takes (e.g., 2 for memory reference – fetch and execute, 1 for operate).
• Indirect Cycles: Each level of indirect addressing requires an additional memory cycle to fetch the effective address.
• Extra Cycles: Some instructions, like multiple shifts in an operate instruction, require additional cycles beyond the base. For the PDP-1, some complex operate instructions or multiple shifts could add time.
• Memory Cycle Time (Tmc): The time for one memory read or write operation (e.g., 5 µs).
• I/O Wait Time (Tw): For IOT instructions, the CPU might wait for a slower I/O device, adding directly to the total time.

For example, a memory reference instruction with one level of indirect addressing would take (2 base + 1 indirect) * 5 µs = 15 µs.

Variable	Meaning	Unit	Typical Range (PDP-1)
Tmc	Memory Cycle Time	µs	5 (core memory)
I	Indirect Addressing Levels	levels	0, 1, 2…
Tw	I/O Device Wait Time	µs	0 – 1000s (device dependent)
S	Number of Shifts/Rotates	count	0, 1, 2…
BaseC	Base Cycles	cycles	1 – 2

Variables used in the PDP-1 Instruction Timing Calculator.

Practical Examples (Real-World Use Cases)

Example 1: Memory Reference with Indirect Addressing

Suppose we want to execute an `add` instruction that uses one level of indirect addressing. The PDP-1 memory cycle time is 5 µs.

• Instruction Type: Memory Reference (Indirect)
• Memory Cycle Time: 5 µs
• Indirect Levels: 1

Base cycles = 2, Indirect cycles = 1. Total cycles = 2 + 1 = 3.
Total Time = 3 * 5 µs = 15 µs.
The PDP-1 Instruction Timing Calculator would show 15 µs.

Example 2: IOT Instruction with Wait

Consider an IOT instruction communicating with a device that requires a 20 µs wait time. Memory cycle time is 5 µs.

• Instruction Type: IOT
• Memory Cycle Time: 5 µs
• I/O Wait Time: 20 µs

Base IOT instruction takes 1 cycle (5 µs) for its own execution phases, plus the wait time.
Total Time = (1 * 5 µs) + 20 µs = 5 µs + 20 µs = 25 µs.
The PDP-1 Instruction Timing Calculator would display 25 µs.

How to Use This PDP-1 Instruction Timing Calculator

1. Select Instruction Type: Choose the type of PDP-1 instruction from the dropdown menu (e.g., Memory Reference, IOT, Operate). The available inputs will adjust based on your selection.
2. Enter Memory Cycle Time: Input the PDP-1’s memory cycle time in microseconds (µs). The default is 5 µs.
3. Specify Indirect Levels (if applicable): If you selected “Memory Reference (Indirect)”, enter the number of indirect addressing levels.
4. Enter I/O Wait Time (if applicable): For “IOT” instructions, input the expected wait time for the I/O device in µs.
5. Enter Shift Count (if applicable): For “Operate Group (with Shifts/Rotates)”, specify the number of single-bit shift/rotate operations.
6. Indicate Skip Taken (if applicable): For “Skip Group”, select whether the skip condition is met (“Yes”) or not (“No”).
7. View Results: The calculator instantly displays the “Total Instruction Time”, “Base Time”, “Indirect Addressing Time”, and “Extra Cycles/Wait Time”, along with the formula used. The chart also updates.
8. Reset or Copy: Use the “Reset” button to go back to default values or “Copy Results” to copy the output.

Understanding the results helps appreciate how different factors influenced the PDP-1’s performance. A longer time means slower execution for that specific instruction instance.

Key Factors That Affect PDP-1 Instruction Timing Results

• Memory Cycle Time: The fundamental factor. Slower memory directly increases the time for every cycle. The original PDP-1 core memory was 5 µs per cycle.
• Instruction Type: Different instructions (memory reference, I/O, operate) have different base cycle requirements. Memory reference typically takes more cycles than simple operate instructions.
• Indirect Addressing: Each level of indirect addressing requires an extra memory cycle to fetch the next address, significantly increasing time for memory reference instructions.
• I/O Device Speed: For IOT instructions, the CPU might have to wait for slower peripheral devices, adding directly to the total execution time. This was a major factor in overall system performance when interacting with devices like teletypes or paper tape readers.
• Operate Instruction Complexity: While many operate instructions were single-cycle, those involving multiple shifts or rotates would take longer as each shift/rotate added time, often related to sub-cycle operations within the main cycle or requiring extra cycles. The PDP-1 Instruction Timing Calculator models this for shifts.
• Skip Condition: For skip instructions, if the condition to skip is met, an extra cycle is usually consumed to bypass the next instruction, affecting program flow and timing.

These factors highlight the variable nature of instruction execution time even on relatively simple early computers like the PDP-1.

Frequently Asked Questions (FAQ)

What was the basic clock speed of the PDP-1?

The PDP-1 didn’t have a “clock speed” in the modern CPU sense. Its operation was timed by the memory cycle time, which was 5 microseconds (5 µs). This means the basic memory operations happened at a rate equivalent to 200 kHz (1 / 5 µs).

Why did indirect addressing take longer?

Each level of indirect addressing required an additional memory read cycle to fetch the address contained at the current memory location before finally getting the operand’s address or the operand itself. The PDP-1 Instruction Timing Calculator adds one memory cycle time for each indirect level.

Did all operate instructions take one cycle?

Many basic operate group instructions (like clear, complement) did execute within one 5 µs cycle. However, instructions involving multiple shifts or rotates could take longer because each single-bit shift/rotate operation added time within or beyond that single cycle.

How did I/O affect timing?

IOT (Input/Output Transfer) instructions could involve significant delays if the CPU had to wait for a slow mechanical device (like a teletype) to be ready or to complete an operation. This wait time was added to the instruction’s execution time.

Is the 5 µs cycle time constant?

For the original core memory of the PDP-1, yes, it was 5 µs. Different memory technologies or later modifications could theoretically have different cycle times, which you can input into the PDP-1 Instruction Timing Calculator.

What is the difference between JMP and JSP?

JMP (Jump) simply changes the program counter. JSP (Jump to Subroutine) also changes the program counter but first saves the old program counter (return address) in a specific memory location or register (in PDP-1, it stored it at the destination address and jumped to destination+1), taking one cycle.

Could the PDP-1 overlap instructions?

No, the PDP-1 was a very early machine and did not have instruction pipelining or other techniques to overlap instruction execution. Instructions were fetched and executed sequentially. This makes the timing calculated by the PDP-1 Instruction Timing Calculator more directly representative.

How does this compare to modern computers?

Modern computers operate millions of times faster, with clock cycles measured in nanoseconds or picoseconds, and complex techniques like pipelining and parallel execution. The PDP-1’s 5 µs cycle time is very slow by today’s standards, but it was fast for its era. See computer performance metrics for more.