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🐛 fix crt0 trap handler
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@stnolting
stnolting committed Feb 17, 2024
1 parent 9ec0358 commit 8f3a2c3
Showing 1 changed file with 16 additions and 15 deletions.
31 changes: 16 additions & 15 deletions sw/common/crt0.S
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
/* # ********************************************************************************************* # */
/* # BSD 3-Clause License # */
/* # # */
/* # The NEORV32 RISC-V Processor, https://github.com/stnolting/neorv32 # */
/* # Copyright (c) 2024, Stephan Nolting. All rights reserved. # */
/* # # */
/* # Redistribution and use in source and binary forms, with or without modification, are # */
Expand All @@ -28,8 +29,6 @@
/* # AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING # */
/* # NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED # */
/* # OF THE POSSIBILITY OF SUCH DAMAGE. # */
/* # ********************************************************************************************* # */
/* # The NEORV32 Processor - https://github.com/stnolting/neorv32 (c) Stephan Nolting # */
/* ################################################################################################# */

.file "crt0.S"
Expand All @@ -49,11 +48,11 @@ __crt0_entry:
// Setup CPU core CSRs
// ************************************************************************************************
__crt0_cpu_csr_init:
li x1, 0x00001800
csrw mstatus, x1 // reset mstatus (e.g. no interrupt in machine-mode)
li x1, 0x00001800 // mstatus.mpp = machine-mode
csrw mstatus, x1
la x1, __crt0_trap_handler // configure early-boot trap handler
csrw mtvec, x1
csrw mie, zero // disable all interrupt sources
csrw mie, zero // disable all interrupt sources


// ************************************************************************************************
Expand All @@ -62,18 +61,20 @@ __crt0_cpu_csr_init:
__crt0_pointer_init:
.option push
.option norelax
// Setup pointers using linker script symbols
la sp, __crt0_stack_begin // stack pointer
la gp, __global_pointer$ // global pointer
// setup pointers using linker script symbols
la x4, __crt0_stack_end // stack pointer
andi sp, x4, 0xfffffffc // word-aligned
la x5, __global_pointer$ // global pointer
andi gp, x5, 0xfffffffc // word-aligned
.option pop

__crt0_reg_file_init:
//addi x0, x0, 0 // hardwired to zero
//addi x1, x0, 0 // implicitly initialized within crt0
//addi x2, x0, 0 // stack pointer sp
//addi x3, x0, 0 // global pointer gp
addi x4, x0, 0
addi x5, x0, 0
//addi x4, x0, 0 // implicitly initialized within crt0
//addi x5, x0, 0 // implicitly initialized within crt0
addi x6, x0, 0
addi x7, x0, 0
addi x8, x0, 0
Expand Down Expand Up @@ -213,20 +214,20 @@ __crt0_trap_handler:
srli x8, x8, 31 // isolate MSB (set for interrupts)
bnez x8, __crt0_trap_handler_end

// mepc = mepc + 2 (for compressed instruction)
// mepc = mepc + 4 (for UNCOMPRESSED instruction)
csrr x8, mepc
addi x8, x8, +2
addi x8, x8, +4
csrw mepc, x8

// exit if exception-causing instruction is compressed
// exit if exception-causing instruction is uncompressed
csrr x8, mtinst // get transformed exception-causing instruction
andi x8, x8, 3 // isolate lowest 2 opcode bits (= 11 for uncompressed instructions)
addi x8, x8, -3 // x8 is zero after this if uncompressed instruction
beqz x8, __crt0_trap_handler_end

// mepc = mepc + 2 (another time; for uncompressed instruction)
// mepc = mepc - 2 (making mepc_new = mepc_old + 2 for COMPRESSED instruction)
csrr x8, mepc
addi x8, x8, +2
addi x8, x8, -2
csrw mepc, x8

// restore x8
Expand Down

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