Wednesday, October 11, 2006
First Post
Well here we are, first post on a journey back into time.
It's been around 20 years since I last touched a real Spectrum to program it, but here I am back again!!!
Mike has given me this chance to put together a Spectrum version of Xenon TriOxide, so I've been re-acquainting myself with an old friend.
The game is a scrolling shoot em up, and the best scrolling method on the Spectrum was used in Cobra, where the screen is redrawn every frame by "push"ing the map onto the screen.
The technique is fairly simple the stack pointer is set to a screen address and then code is executed which draws the screen, this code is created procedurally from the game map, it is a simple tower of push instructions, in psuedo code it is reasonably simple
Tomorrow I'll talk about how to synchronise the screen so that we don't get any flicker.
It's been around 20 years since I last touched a real Spectrum to program it, but here I am back again!!!
Mike has given me this chance to put together a Spectrum version of Xenon TriOxide, so I've been re-acquainting myself with an old friend.
The game is a scrolling shoot em up, and the best scrolling method on the Spectrum was used in Cobra, where the screen is redrawn every frame by "push"ing the map onto the screen.
The technique is fairly simple the stack pointer is set to a screen address and then code is executed which draws the screen, this code is created procedurally from the game map, it is a simple tower of push instructions, in psuedo code it is reasonably simple
- disable interrupts
- store SP into memory location (of the ld sp,NNNN instruction below)
- set SP to screen address (right most column of screen)
- initialises registers with the graphics
- Execute the constructed code
- Loop for all the screen lines that are to be rendered
- ld sp,NNNN (this instruction will now contain the stored SP register)
- enable interrupts
Tomorrow I'll talk about how to synchronise the screen so that we don't get any flicker.
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Thanks for the encouragement :)
No graphics just yet as I've a lot of code to do before we get to that stage.
I'll keep you posted thoug
No graphics just yet as I've a lot of code to do before we get to that stage.
I'll keep you posted thoug
Go Russell go! I wanna see how it will run, the early stages too! Occasionally, turn the chair and slap the other fruit that is coding the other two versions! :D
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