86 lines
2.8 KiB
Ruby
86 lines
2.8 KiB
Ruby
module Arm
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# There are only three call instructions in arm branch (b), call (bl) and syscall (swi)
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# A branch could be called a jump as it has no notion of returning
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# The pc is put into the link register to make a return possible
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# a return is affected by moving the stored link register into the pc, effectively a branch
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# swi (SoftWareInterrupt) or system call is how we call the kernel.
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# in Arm the register layout is different and so we have to place the syscall code into register 7
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# Registers 0-6 hold the call values as for a normal c call
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class CallInstruction < Register::Branch
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include Constants
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include Attributed
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def initialize(first, attributes)
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super(nil, nil)
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@attributes = attributes
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raise "no target" if first.nil?
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@first = first
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opcode = @attributes[:opcode].to_s
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if opcode.length == 3 and opcode[0] == "b"
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@attributes[:condition_code] = opcode[1,2].to_sym
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@attributes[:opcode] = :b
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end
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if opcode.length == 6 and opcode[0] == "c"
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@attributes[:condition_code] = opcode[4,2].to_sym
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@attributes[:opcode] = :call
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end
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@attributes[:update_status] = 0
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@attributes[:condition_code] = :al if @attributes[:condition_code] == nil
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end
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def assemble(io)
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case @attributes[:opcode]
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when :b, :call
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handle_call(io)
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when :swi
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handle_swi(io)
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else
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raise "Should not be the case #{inspect}"
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end
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end
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def handle_call(io)
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case @first
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when Register::Label
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# relative addressing for jumps/calls
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# but because of the arm "theoretical" 3- stage pipeline,
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# we have to subtract 2 words (fetch/decode)
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arg = arg.position - self.position - 8
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when Parfait::TypedMethod
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# But, for methods, this happens to be the size of the object header,
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# so there it balances out, but not blocks
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# have to use the code, not the mthod object for methods
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arg = arg.binary.position - self.position
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else
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arg = @first
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end
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write_call(arg, io)
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end
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def write_call(arg,io)
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raise "else not Attributed arg =\n#{arg.to_s[0..1000]}: #{inspect[0..1000]}" unless (arg.is_a?(Numeric))
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jmp_val = arg >> 2
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packed = [jmp_val].pack('l')
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# signed 32-bit, condense to 24-bit
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# TODO add check that the value fits into 24 bits
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io << packed[0,3]
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io.write_uint8 op_bit_code | (COND_CODES[@attributes[:condition_code]] << 4)
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end
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def handle_swi(io)
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arg = @first
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raise "expected literal not #{arg} #{inspect}" unless (arg.is_a?(Numeric))
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packed = [arg].pack('L')[0,3]
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io << packed
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io.write_uint8 0b1111 | (COND_CODES[@attributes[:condition_code]] << 4)
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end
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def to_s
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"#{opcode} #{@first} #{super}"
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end
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end
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end
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