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ZX Spectrum 16K/48K: Difference between revisions
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Much of the ZX Spectrum's custom logic is contained within an Uncommitted Logic Array and was designed by [[Richard Altwasser]]. The ULA was manufactured by [[Ferranti]]. ''The ZX Spectrum ULA'' by [[Chris Smith]] describes this device in detail. | Much of the ZX Spectrum's custom logic is contained within an Uncommitted Logic Array and was designed by [[Richard Altwasser]]. The ULA was manufactured by [[Ferranti]]. ''The ZX Spectrum ULA'' by [[Chris Smith]] describes this device in detail. | ||
The ULA suffers from a few oversights in its design and implementation. An error in the timings applied by the ULA's [[I/O contention]] circuit required a modification, the "dead cockroach", such that all I/O access to the I/O port that the ULA provides is contended as though the access is to the lower 16K of RAM, for which access is shared with the ULA. | The ULA suffers from a few oversights in its design and implementation. An error in the timings applied by the ULA's [[Contended I/O|I/O contention]] circuit required a modification, the "dead cockroach", such that all I/O access to the I/O port that the ULA provides is contended as though the access is to the lower 16K of RAM, for which access is shared with the ULA. | ||
The "dead cockroach" was incorporated into later revisions of the ULA, rather than fixing the timing issue. The reason for this is not clear. | The "dead cockroach" was incorporated into later revisions of the ULA, rather than fixing the timing issue. The reason for this is not clear. | ||
