Class: Zemu::Config::Timer

Inherits:
BusDevice show all
Defined in:
lib/zemu/config.rb

Overview

Non-Maskable Interrupt Timer

Represents a timer device, the period of which can be controlled by the CPU through an IO port. The timer generates an NMI once this period has expired. The timer can be reset via a control port.

Constant Summary collapse

RUNNING =

Timer is running - count decrements once per clock cycle.

0x01
STOPPED =

Timer is stopped - count does not change on clock cycle.

0x00

Instance Method Summary collapse

Methods inherited from BusDevice

#functions, #interrupt, #interrupt?, #io_read, #mem_read, #mem_write, #memory, #nmi, #nmi?, #when_setup

Methods inherited from Zemu::ConfigObject

#method_missing

Constructor Details

#initialize ⇒ Timer

Returns a new instance of Timer.



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# File 'lib/zemu/config.rb', line 681

def initialize
    super

    @count = 0
    @running = false
end

Dynamic Method Handling

This class handles dynamic methods through the method_missing method in the class Zemu::ConfigObject

Instance Method Details

#clock(cycles) ⇒ Object

Clock handler.

Handles a clock cycle for this device. Sets NMI active if the count reaches 0.



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# File 'lib/zemu/config.rb', line 706

def clock(cycles)
    if @running == RUNNING
        if @count > 0
            @count -= cycles
        else
            nmi(true)
        end
    end
end

#io_write(port, value) ⇒ Object

IO bus write handler.

Handles write access via the IO bus to this device.

Parameters:

  • port —

    The IO port being accessed.

  • value —

    The value being written.



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# File 'lib/zemu/config.rb', line 694

def io_write(port, value)
    if port == count_port
        @count = value
    elsif port == control_port
        @running = if value == 0 then STOPPED else RUNNING end
    end
end

#params ⇒ Object

Valid parameters for a Timer, along with those defined in [Zemu::Config::IOPort].



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# File 'lib/zemu/config.rb', line 718

def params
    super + %w(count_port control_port)
end