EMBARK/kinclude/boot.S

#include "csr.h"
.section    .stack

stack_bottom:
.space      4096
stack_top:

.section    .text._start

.extern     init
.type       init, @function

.extern     trap_handle
.type       trap_handle, @function


.global     _start
_start:
            // enable interrupts in mstatus
            // this is the setting loaded:
            // [07]    MPIE    =   1  - we want to enable interrupts with mret
            // [03]    MIE     =   0  - we don't want interrupts now
            // [11:12] MPP     =   0  - we want to return into user mode
            // all other bits should be zero
            li      a0, 0x80
            csrrw   zero, CSR_MSTATUS, a0         // write to mstatus
            // setup a0 to hold |trap tbl addr|mode|
            //              len:| 30          | 2  |
            la      a0, trap_vector
            csrrw   zero, CSR_MTVEC, a0         // write a0 into mtvec csr entry
            // write
.option push
.option norelax
            // init sp and gp
            la      sp, stack_top
            la      gp, _gp
.option pop
            // clear kernel bss section
            mv      a0, zero
            la      a1, _bss_start
            la      a2, _bss_end
            jal     memset

            // jump to init
            jal     init
            
            // halt machine after returning from init
            csrwi   CSR_HALT, 1
1:
            j       1b

.align 4
trap_vector:
            // save all registers into the PCB struct
            // switch contents of t6 with contents of mscratch
            // mscratch holds the PCBs regs field address
            csrrw   t6,  CSR_MSCRATCH, t6
            sw      ra,  0(t6)
            sw      sp,  4(t6)
            sw      gp,  8(t6)
            sw      tp,  12(t6)
            sw      t0,  16(t6)
            sw      t1,  20(t6)
            sw      t2,  24(t6)
            sw      s0,  28(t6)
            sw      s1,  32(t6)
            sw      a0,  36(t6)
            sw      a1,  40(t6)
            sw      a2,  44(t6)
            sw      a3,  48(t6)
            sw      a4,  52(t6)
            sw      a5,  56(t6)
            sw      a6,  60(t6)
            sw      a7,  64(t6)
            sw      s2,  68(t6)
            sw      s3,  72(t6)
            sw      s4,  76(t6)
            sw      s5,  80(t6)
            sw      s6,  84(t6)
            sw      s7,  88(t6)
            sw      s8,  92(t6)
            sw      s9,  96(t6)
            sw      s10, 100(t6)
            sw      s11, 104(t6)
            sw      t3,  108(t6)
            sw      t4,  112(t6)
            sw      t5,  116(t6)
            mv      a0,  t6             // save struct address to already saved register
            csrrw   t6,  CSR_MSCRATCH, t6   // load original t6 register from mscratch
            sw      t6,  120(a0)        // save original t6 register
            // save mepc to pc field in pcb
            csrr    t6,  CSR_MEPC
            sw      t6,  -4(a0)
            // load mcause and mtval values in the correct registers for call to trap_handle function
            csrr    a1,  CSR_MCAUSE
            srli    a0,  a1, 31
            slli    a1,  a1, 1
            srli    a1,  a1, 1
            csrr    a2,  CSR_MTVAL
            // reinit sp and gp
.option push
.option norelax
            la      sp, stack_top
            la      gp, _gp
.option pop
            jal     trap_handle


// make memset global
.global     memset
.type       memset, @function
#ifdef __risc_no_ext
// "dumb" memset, if RV32M is not present on the target
// since memset is currently only used at startup, the performance implications
// should be minimal.
memset:
            bge     a1, a2, 2f
1:
            sw      a0, 0(a1)
            addi    a1, a1, 4
            blt     a1, a2, 1b
2:
            ret

#else

// "smart" memset, writing 32 bytes at a time. uses RV32M. If not present,
// the "dumb" fallback above is used.

// write a0 to memory starting at a1, until a2 (both must be four byte aligned)
// this uses a loop which writes 32 (numbytes) bytes at a time
// to prevent overshooting the end, we first calulate how many instructions to 
// skip of the first iteration of the loop. this way, (a2 - a1) is a multiple of
// (numbytes) when we reach the blt instruction for the first time. 
// this math works so good, because we write 4 bytes of mem, in 4 bytes of 
// instructions. Therefore instruction bytes to skip = write bytes to skip
// bytes to skip = numbytes - ((a2 - a1) % numbytes)
memset:
            sub     t1, a2, a1      // t1 = a2 - a1
            li      t2, 32          // = numbytes
            rem     t1, t1, t2      // t1 = (a2 - a1) % numbytes
            beq     zero, t1, 1f    // skip 0 bytes? => begin loop
            sub     t2, t2, t1      // t2 = numbytes - ((a2 - a1) % numbytes)
                                    // = bytes to skip
            sub     a1, a1, t2      // subtract skipped bytes from a2
                                    // to account for the skipped instruction
                                    // when we reach the addi, a1, a1, 32 inst.
            auipc   t1, 0           // get current address
            add     t1, t2, t1      // add calulated offset
            jalr    zero, t1, 12    // skip the instructions by forward-jumping
                                    // the 12 is added to compensate for the 
                                    // three instructions auipc, add, jalr
1:
            sw      a0, 0(a1)
            sw      a0, 4(a1)
            sw      a0, 8(a1)
            sw      a0, 12(a1)
            sw      a0, 16(a1)
            sw      a0, 20(a1)
            sw      a0, 24(a1)
            sw      a0, 28(a1)
            addi    a1, a1, 32
            blt     a1, a2, 1b
            ret
#endif