| /* |
| * Copyright (c) 2016 Jean-Paul Etienne <fractalclone@gmail.com> |
| * |
| * SPDX-License-Identifier: Apache-2.0 |
| */ |
| |
| #include <toolchain.h> |
| |
| /* imports */ |
| GTEXT(__reset) |
| GTEXT(__irq_wrapper) |
| |
| /* |
| * following riscv32-qemu specs |
| * IVT is placed at 0x000001000 and is mapped as follows: |
| * 0x00001000: reset |
| * 0x00001004: non-maskable interrupt (nmi) vector |
| * 0x00001010: machine trap (mt) vector |
| * |
| * Call __irq_wrapper to handle all interrupts/exceptions/faults |
| */ |
| SECTION_FUNC(vectors, vinit) |
| .option norvc; |
| |
| /* |
| * jal instruction cannot be used to jump to address whose offset |
| * is > 12-bits wide. In this case, we have to use a call or tail |
| * instruction to jump to a far-away sub-routine. |
| * |
| * Given that IVT is found at a different address-space than the |
| * RAM in riscv32-qemu, we have to use call or tail instructions |
| * to jump to __reset or __isr_wrapper subroutines. |
| * However, call or tail instructions are pseudo instructions, |
| * which generate two base-instructions upon compilation. In this case, |
| * using them at a particular entry in the IVT will overwrite the next |
| * entry in the IVT. For example, using tail instruction in the |
| * reset vector, will overwrite the nmi-vector entry. To prevent this, |
| * perform a two-phase jump instructions to __reset or __irq_wrapper |
| * subroutines. The first jump performs a jal instruction, which will |
| * jump to an offset in the same vector address-space, but outside the |
| * IVT. The second jump performs a tail instruction to the __reset |
| * or __irq_wrapper subroutines. |
| */ |
| |
| /* Call __reset for reset vector */ |
| jal x0, do_reset |
| |
| /* Call __irq_wrapper for nmi vector */ |
| jal x0, do_irq_wrapper |
| |
| .org 0x10 |
| /* Call __irq_wrapper for mt vector */ |
| jal x0, do_irq_wrapper |
| |
| .org 0x400 /* we are outside IVT */ |
| do_reset: |
| /* |
| * Set mtvec (Machine Trap-Vector Base-Address Register) |
| * to __irq_wrapper, so that we jump directly to __irq_wrapper, |
| * instead to the default machine trap vector address in IVT. |
| * This will preserve us from performing two jump instructions upon |
| * an interrupt. |
| */ |
| la t0, __irq_wrapper |
| csrw mtvec, t0 |
| |
| /* Jump to __reset */ |
| tail __reset |
| |
| do_irq_wrapper: |
| tail __irq_wrapper |
