72 lines
1.3 KiB
ArmAsm
72 lines
1.3 KiB
ArmAsm
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# The main body of code loops on the variable FLAG, waiting
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# for it to be set. When set, the message `Tick' is displayed
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# and the flag cleared.
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# The interrupt handler, sets the global FLAG variable and
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# then resumes normal execution.
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.globl _start
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.ent _start
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.set noreorder
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_start:
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# Patch the interrupt handler, setting it to an instruction
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# sequence that jumps to this programs interrupt code.
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la $4, handler
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la $5, +0xffffffffBFC00200 + 0x180
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lw $6, 0($4) # BR $8
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sw $6, 0($5)
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lw $6, 4($4) # NOP
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sw $6, 4($5)
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# establish two global pointer registers:
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la $8, flag # r8 contains the address of FLAG
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la $9, real_handler # r9 contains the address of the
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# real handler
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# enable interrupt delivery
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mfc0 $4, $12
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nop ; nop ; nop
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or $4, $4, 1
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or $4, $4, 4 ; xor $4, $4, 4 # clear error level
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mtc0 $4, $12
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nop ; nop ; nop
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loop:
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lw $5, 0($8)
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beq $5, $0, loop
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nop
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# FLAG set, print the message Tick by calling
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# write (1, str, 6)
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sw $0, 0($8)
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li $4, 1
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la $5, str
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li $6, 6
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la $2,+0xffffffffbfc00504
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lw $2, 0($2)
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jal $2
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nop
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j loop
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nop
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# Instruction sequence that jumps to
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# the real handler (address in r9).
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handler: j $9
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nop
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# local interrupt handler, set FLAG and return
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real_handler:
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li $10, 1
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sw $10, 0($8) # flag = 1
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eret
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nop
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.end _start
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flag:
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.word 0
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str: .asciiz "Tick\r\n"
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