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Floating bus: Difference between revisions
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When the Z80 reads from an unattached port it will read the data present on the ULA bus, which will be a display byte being transferred to the video circuits or 0xFF when idle, such as periods spent building the border. | When the Z80 reads from an unattached port it will read the data present on the ULA bus, which will be a display byte being transferred to the video circuits or 0xFF when idle, such as periods spent building the border. | ||
Each scanline of video memory fetches breaks down into a | Each scanline of video memory fetches breaks down into a 16×8 cycle sequence with two sets of display and attribute bytes (order: bitmap, attribute, bitmap+1, attribute+1) being fetched during the first 4 cycles followed by 4 idle cycles. The ULA bus remains idle for the remainder of each scanline and returns 0xFF. | ||
The following table shows the fetch cycles for the first 8 cycle sequence of the 48K and 128K models<ref>This document labels the first T-state which ''begins'' with | The following table shows the fetch cycles for the first 8 cycle sequence of the 48K and 128K models<ref>This document labels the first T-state which ''begins'' with {{overline|INT}} low as T-state 0; some other resources label this as T-state 1 which will mean all T-state counts are one greater.</ref>: | ||
{| class="wikitable" | {| class="wikitable" | ||
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A number of commercial games used the floating bus effect, generally to synchronise with the display and allow for flicker-free drawing. Games known to use the floating bus include: | A number of commercial games used the floating bus effect, generally to synchronise with the display and allow for flicker-free drawing. Games known to use the floating bus include: | ||
* ''[[wikipedia:Arkanoid|Arkanoid]]'' (original release only, the Hit Squad | * ''[[wikipedia:Arkanoid|Arkanoid]]'' (original release only, the Hit Squad re-release does not use the floating bus) | ||
* ''[[wikipedia:Cobra (film)#Video game|Cobra]]'' (original release only, the Hit Squad | * ''[[wikipedia:Cobra (film)#Video game|Cobra]]'' (original release only, the Hit Squad re-release does not use the floating bus) | ||
* ''[[wikipedia:Sidewize|Sidewize]]'' | * ''[[wikipedia:Sidewize|Sidewize]]'' | ||
* [[wikipedia:Short_Circuit|Short Circuit]] | * [[wikipedia:Short_Circuit|Short Circuit]] | ||