ddcput is a micro-processor testing utility that can:

• Measure user-code instruction latency.

[1] Until I get a macOS VM or someone willing to implement the necessities

(x86/amd64) This tool is NOT supported in any 16-bit modes, such as real and vm8086 modes.

For the cycle count, the latency mode will perform an average by arithmetic mean, meaning it only takes two timestamps (RDTSC) and divides the different by the number of times it looped.

Currently, the latency tester will run instructions depending on the compiled binary. For example, the amd64 build will only run amd64 code while the x86 builds will run x86 code. A feature to run x86 code in amd64 is planned.

• Binary code (file) must be flat (i.e. -fbin on NASM);
• It MUST NOT return or jump beyond your code;
• Binary must fit in a single page (4096 Bytes - size_t.sizeof);
• And the flow must reach past the end of your code.
  • The tool inserts its own pre-test and post-test code.

This is a "for fun" mode. Can be used to get a general idea of available registers on the platform.

x86 is a complex ISA, it might happen that assemblers do not know the full context of the source code and may generate improper code.

In x86 ("long compability mode"), NASM may prefix a MOV instruction with an OPCODE PREFIX (66h), even if BITS 32 is specified (no effects in BITS 64).

The OPCODE prefix serves as inverting 16-bit and 32-bit instructions, useful for executing 32-bit instructions in 16-bit mode (real-address mode) to transition to protected-mode, but when used improperly, may result in serious bugs.

For example, a mov eax, 0 instruction should be encoded like so (x86-32):

B8 00 00 00 00

However, if the instruction is encoded like so:

66 B8 00 00 00 00

It results in mov ax, 0, and the processor will move a WORD (2 bytes) instead of a DWORD (4 bytes) and move the Instruction Pointer to the next two bytes: 00 00h, which is encoded like add [eax], al, and since EAX=0 in this case, it will attempt to write at address 0h and effectively segfault (#UD).

Please note that some emulators and hypervisors, such as Oracle VirtualBox, may improperly emulate the RDTSC instruction, since the RDTSC instruction is used in the latency mode for its cycle count.

SINCE THIS TOOL DIRECTLY RUNS USER GENERATED BINARY CODE, THIS TOOL IS ENTIRELY UNSECURE. MAKE SURE THIS TOOL IS NOT AVALAIBLE WITHIN YOUR PATH FOR SECURITY REASONS. NEVER RUN CODE THAT YOU DO NOT TRUST (i.e. COMPILED BY ANOTHER PERSON, SUCH AS ONLINE EXAMPLES).

LATENCY RESULTS ARE BASED ON THE NUMBER OF TIMES THEY RAN (AVERAGE BY ARITHMETIC MEAN), THE HIGHER THE COUNT, THE HIGHER PRECISION, BUT DOES NOT GARANTEE ABSOLUTE METRICS.

See LICENSE for more information.