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.section .stack
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stack_bottom:
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.space 4096
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stack_top:
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.section .text._start
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// Set up all the CSR mstatus_offsets
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.set mstatus, 0x300 // machine status
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.set mscratch, 0x340
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.set mtvec, 0x305 // machine trap handler
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.set mcause, 0x342 // machine trap cause
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.set mtval, 0x343 // machine bad address or instruction
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// .set misa, 0x301 // machine ISA
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// .set mie, 0x304 // machine interrupt enable
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// .set mip, 0x344 // machine interrupt pending
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.extern init
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.type init, @function
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.extern trap_handle
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.type trap_handle, @function
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.global _start
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_start:
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// setup a0 to hold |trap tbl addr|mode|
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// len:| 30 | 2 |
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la a0, trap_vector
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csrrw zero, mtvec, a0 // write a0 into mtvec csr entry
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// enable interrupts in mstatus
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// this is the setting loaded:
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// [07] MPIE = 1 - we want to enable interrupts with mret
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// [03] MIE = 0 - we don't want interrupts now
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// [11:12] MPP = 0 - we want to return into user mode
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// all other bits should be zero
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li a0, 0x80
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csrrw zero, mstatus, a0 // write to mstatus
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// write
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.option push
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.option norelax
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// init sp and gp
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la sp, stack_top
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la gp, __global_pointer$
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.option pop
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jal init
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// halt machine
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.align 4
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trap_vector:
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// save all registers into the PCB struct
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// switch contents of t6 with contents of mscratch
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// mscratch holds the PCBs regs field address
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csrrw t6, mscratch, t6
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sw ra, 0(t6)
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sw sp, 4(t6)
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sw gp, 8(t6)
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sw tp, 12(t6)
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sw t0, 16(t6)
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sw t1, 20(t6)
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sw t2, 24(t6)
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sw s0, 28(t6)
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sw s1, 32(t6)
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sw a0, 36(t6)
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sw a1, 40(t6)
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sw a2, 44(t6)
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sw a3, 48(t6)
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sw a4, 52(t6)
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sw a5, 56(t6)
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sw a6, 60(t6)
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sw a7, 64(t6)
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sw s2, 68(t6)
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sw s3, 72(t6)
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sw s4, 76(t6)
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sw s5, 80(t6)
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sw s6, 84(t6)
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sw s7, 88(t6)
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sw s8, 92(t6)
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sw s9, 96(t6)
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sw s10, 100(t6)
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sw s11, 104(t6)
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sw t3, 108(t6)
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sw t4, 112(t6)
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sw t5, 116(t6)
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mv a0, t6 // save struct address to already saved register
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csrrw t6, mscratch, t6 // load original t6 register from mscratch
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sw t6, 120(a0) // save original t6 register
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csrr a1, mcause
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srli a0, a1, 31
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slli a1, a1, 1
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srli a1, a1, 1
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csrr a2, mtval
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// reinit sp and gp
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.option push
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.option norelax
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la sp, stack_top
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la gp, __global_pointer$
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.option pop
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jal trap_handle
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