Module layout 

PVM2 guest virtual-address-space layout (ABI constants).

These constants define where a transpiler-emitted Image’s code and data regions map in the guest’s 32-bit address space. They are part of the PVM2 ABI contract: the transpiler (javm-transpiler) bakes PC = CODE_BASE + byte_offset into endpoint entry PCs and native auipc/jalr resolution and lays data caps from DATA_BASE up, and every runtime (nub-arch-x86, nub-arch-local, javm) maps Image.code read-only at CODE_BASE and data at DATA_BASE.

The constants live here in javm-cap because it is the only crate every producer (transpiler) and consumer (each runtime) depends on. Code placement is a fixed protocol constant rather than an Image-supplied mapping entry: an untrusted Image must not get to choose where its code lands.

  [0,         CODE_BASE)  unmapped — NULL guard (catch PC=0 / null deref)
  [CODE_BASE, DATA_BASE)  CODE     — RO, ≤ MAX_CODE_SIZE bytes
  [DATA_BASE, 4 GiB)      DATA     — stack / ro / rw / heap, RO|RW

Code low (4 MiB) gives the null guard; data high (256 MiB) keeps the whole data region contiguous above code instead of wrapping around it. Both [0, CODE_BASE) and [CODE_BASE + code, DATA_BASE) are unmapped, so a stray fetch or load there faults.

Constants

CODE_BASE

Guest virtual address where the (single) code region maps read-only. A PVM PC is CODE_BASE + byte_offset. Sits at 4 MiB so [0, 4 MiB) is an unmapped null guard.

DATA_BASE

Guest virtual address where the data region begins. All data caps (stack / ro / rw / heap) and instance overlays live in [DATA_BASE, 4 GiB). At 256 MiB, well clear of the largest permitted code region.

MAX_CODE_SIZE

Maximum byte length of the code region. Code occupies [CODE_BASE, CODE_BASE + code_len) and must stay below DATA_BASE, so code_len ≤ DATA_BASE − CODE_BASE = 252 MiB.