Documentation
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Overview ¶
Package polycounter implements the polynomial counters found in the TIA. Described by Andrew Towers in the "Atari 2600 TIA Hardware Notes" (TIA_HW_Notes.txt), polynomial counters are a predictably performative way of counting in simple electronics - performance of ripple counters can change due to carrying etc.
In our emulation we are normal integers but for the purposes of debugging the TIA loop (HSYNC counter) we'd still like to know what the equivalent polycounter value is. We use a 6-bit polycounter for this.
hsync := polycounter.New(6)
As the emulated polycounter is just an integer we can "tick" it along in the obvious way. We should take care to make sure it doesn't run past the end of the polycounter however. The accepted pattern is:
p++
if p >= polycounter.LenTable6Bit {
p = 0
}
Whenever the polycounter is to be reset set it it polycounter.ResetValue.
The polycounter bit pattern can be retrieved at any time with the ToBinary() function.
Additional Note ¶
In the 2600, polycounter logic is also used to generate the bit sequences required for TIA audio emulation. A real TIA variously uses 4-bit, 5-bit and 9-bit polycounters to generate the sound waves available to the 2600 programmer. As of yet, this package doesn't support this functionality correctly. The bit sequences required are hard-coded into the tia/audio package as discovered by Ron Fries.
Index ¶
Constants ¶
const LenTable6Bit = 64
number of entries in 6 bit polycounter table.
const ResetValue = 0
ResetValue is used to reset the polycounter.
Variables ¶
This section is empty.
Functions ¶
This section is empty.
Types ¶
type Polycounter ¶
type Polycounter int
Polycounter counts through the entries of a 6 bit polycounter. For the purposes of the emulation we represent it as an integer and index a pre-calculated table as required.
func (Polycounter) String ¶
func (p Polycounter) String() string
func (*Polycounter) ToBinary ¶
func (p *Polycounter) ToBinary() string
ToBinary returns the bit pattern of the current polycounter value.
Source Files