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/* ################################################################################################# */
/* # << NEORV32 - park_loop.S - Execution-Based On-Chip Debugger - Park Loop Code >>               # */
/* # ********************************************************************************************* # */
/* # BSD 3-Clause License                                                                          # */
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/* # Copyright (c) 2021, Stephan Nolting. All rights reserved.                                     # */
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/* # The NEORV32 Processor - https://github.com/stnolting/neorv32              (c) Stephan Nolting # */
/* ################################################################################################# */

// debug memory address map
.equ DBMEM_CODE_BASE, 0xfffff800 // base address of dbmem.code_memory
.equ DBMEM_PBUF_BASE, 0xfffff880 // base address of dbmem.program_buffer
.equ DBMEM_DBUF_BASE, 0xfffff900 // base address of dbmem.data_buffer
.equ DBMEM_SREG_BASE, 0xfffff980 // base address of dbmem.status_register

// status register (SREG) bits
.equ SREG_HALTED_ACK,    (1<<0) // -/w: CPU is halted in debug mode and waits in park loop
.equ SREG_RESUME_REQ,    (1<<1) // r/-: DM requests CPU to resume
.equ SREG_RESUME_ACK,    (1<<2) // -/w: CPU starts resuming
.equ SREG_EXECUTE_REQ,   (1<<3) // r/-: DM requests to execute program buffer
.equ SREG_EXECUTE_ACK,   (1<<4) // -/w: CPU starts to execute program buffer
.equ SREG_EXCEPTION_ACK, (1<<5) // -/w: CPU has detected an exception

.file   "park_loop.S"
.section .text
.balign 4
.option norvc
.global _start
.global entry_normal
.global entry_exception


_start:

// BASE + 0: entry for ebreak in debug-mode, halt request or return from single-stepped instruction
entry_normal:
    jal    zero, parking_loop_start

// BASE + 4: entry for exceptions - signal EXCEPTION to DM and restart parking loop
entry_exception:
    csrw   dscratch0, s0                    // save s0 to dscratch0 so we have a general purpose register available
    addi   s0, zero, SREG_EXCEPTION_ACK     // mask exception acknowledge flag
    sw     s0, DBMEM_SREG_BASE(zero)        // trigger exception acknowledge to inform DM
    csrr   s0, dscratch0                    // restore s0 from dscratch0
    ebreak                                  // restart parking loop

// "parking loop": endless loop that polls the status register to check if the DM
// wants to execute code from the program buffer or to resume normal CPU/application operation
parking_loop_start:
    csrw   dscratch0, s0                    // save s0 to dscratch0 so we have a general purpose register available
    addi   s0, zero, SREG_HALTED_ACK
    sw     s0, DBMEM_SREG_BASE(zero)        // ACK that CPU has halted

parking_loop:
    lw     s0, DBMEM_SREG_BASE(zero)        // get status register
    andi   s0, s0, SREG_EXECUTE_REQ         // request to execute program buffer?
    bnez   s0, execute_progbuf              // execute program buffer

    lw     s0, DBMEM_SREG_BASE(zero)        // get status register
    andi   s0, s0, SREG_RESUME_REQ          // request to resume?
    bnez   s0, resume                       // resume normal operation

    jal    zero, parking_loop               // restart parking loop polling

// resume normal operation
resume:
    addi   s0, zero, SREG_RESUME_ACK
    sw     s0, DBMEM_SREG_BASE(zero)        // ACK that CPU is about to resume
    csrr   s0, dscratch0                    // restore s0 from dscratch0
    dret                                    // end debug mode

// execute program buffer
execute_progbuf:
    addi   s0, zero, SREG_EXECUTE_ACK
    sw     s0, DBMEM_SREG_BASE(zero)        // ACK that execution is about to start
    csrr   s0, dscratch0                    // restore s0 from dscratch0
    fence.i                                 // synchronize i-cache & prefetch with memory (program buffer)
    jalr   zero, zero, %lo(DBMEM_PBUF_BASE) // jump to beginning of program buffer