mcpp.toml

# openarch's armv7a backend: instructions, and nothing else.
#
# THIS PACKAGE EXPORTS NO MODULE. It defines the C entry points declared in
# `abi/include/openarch/abi.h` and is reached only through them, exactly as the
# other four backends are.
#
# THE MACHINE THAT ANSWERED THE WIDTH QUESTION.
#
# openarch was designed on three application-class 64-bit machines, and its
# page-table interface carried an entry in an `arch_u64` without ever saying how
# one is stored — `pte_encode.h` recorded the assumption as a statement of fact:
# "a page-table entry is 64 bits on every machine here". ARMv7-A is the first
# 32-bit machine here WITH an address space, and its short-descriptor entry is
# 32 bits. The value fits; the storage does not. `arch_pte_entry_bytes()` was
# added for this backend and is implemented by all five.
#
# Cortex-M did not settle it and could not: M-profile has an MPU and no page
# table, so its pte group exists and refuses. A 32-bit machine WITHOUT paging
# leaves the question exactly where it was.
[package]
namespace   = "mcpplibs"
name        = "openarch-armv7a"
version     = "0.9.0"
description = "openarch's armv7a backend: the instructions behind the ABI, on the first machine here whose page-table entry is 32 bits"
license     = "Apache-2.0"
authors     = ["mcpplibs"]
repo        = "https://github.com/mcpplibs/openarch"

# `openarch:preemption` IS WITHHELD, AND THAT IS A STATEMENT ABOUT THIS MACHINE
# RATHER THAN AN OMISSION.
#
# `arch_trap_switch` asks a trap to resume a different context. On riscv64 and
# aarch64 the resumption address is a register the dispatcher saves and restores
# around the switch; on Cortex-M a dedicated exception performs it. On ARMv7-A
# it is written to the SVC stack by `srs`, so switching stacks mid-trap changes
# which return frame is popped — well-defined only if the resumed context was
# suspended through the same path. That is a real design, and it is not one
# this backend has measured. A consumer that needs preemption is therefore
# refused by name at resolution rather than linking against something that
# compiles and misbehaves.
#
# This is the mechanism the Cortex-M backend already uses to withhold
# `openarch:address-space`, applied to a different group.
provides    = ["openarch-backend",
               "openarch:address-space",
               "openarch:percpu-register"]

[build]
sources = ["src/**"]

[targets.openarch-armv7a]
kind = "lib"

[dependencies]
openarch-abi = { path = "../../abi" }