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Memory Map Reference

This appendix describes the virtual memory map used by sBPF programs, detailing the memory regions, their purposes, and access permissions.

Memory Regions

sBPF programs operate within a fixed virtual memory map with the following standard regions:

Region Start Address Usable Size Limit Reserved VA Size Access Description
Program Code 0x100000000 ~32 KiB¹ 32MB Read-only Program instructions & read-only data (.rodata) loaded by the VM
Stack 0x200000000 32 KiB 32MB Read/Write Stack memory, grows downward
Heap 0x300000000 Variable 32MB Read/Write Dynamically allocated memory via syscalls
Input Parameters 0x400000000 Variable 32MB Read-only Program input buffer, size depends on transaction

¹ Program size is limited by instruction count (max 4096 instructions = 32 KiB). Read-only data is typically placed within this segment by the linker. Each region is assigned a large virtual address space (e.g., 32MB) for isolation, but the functional limits are much smaller.

Memory Access Rules

  • Accessing memory outside permitted regions or exceeding usable limits results in an AccessViolation error.
  • The stack pointer (fp, register R10) initially points one byte past the end of the usable stack region (e.g., 0x200008000 if stack is 32KiB).
  • Memory accesses should be properly aligned (e.g., 8-byte accesses should be 8-byte aligned) to avoid performance penalties or potential errors.
  • In SBPFv2, the add_stk instruction can be used to modify the stack pointer directly.

Memory Layout Diagram (Conceptual)

+-----------------------+ <- VA End (Higher Addresses)
|                       |
|    Input Parameters   |
|      (Read-only)      |
|                       |
+-----------------------+ 0x400000000
|                       |
|         Heap          |
|      (Read/Write)     |
|      (Variable size)  |
+-----------------------+ 0x300000000
|                       |
|         Stack         |
|      (Read/Write)     |
|       (32 KiB)        |
+-----------------------+ 0x200000000
|                       |
|     Program Code      |
|  & Read-only Data     |
|      (~32 KiB)        |
+-----------------------+ 0x100000000
|                       |
|      Unused           |
|                       |
+-----------------------+ 0x000000000 <- VA Start

(Note: Diagram shows conceptual layout; exact boundaries depend on usable sizes.)

Stack Frame Layout

When an internal function call occurs, the VM pushes a stack frame containing:

  1. Caller-saved registers (R6-R9)
  2. The calling function's frame pointer (FP)
  3. The return address (PC + 1)

In SBPFv2, the VM implicitly reserves a fixed amount of stack space (e.g., 256 bytes) per call. In SBPFv1, stack frame management might involve explicit stack pointer adjustments.

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