Printout routines - MDFS::Info.Comp.Spectrum.ProgTips.FastScreen

Fast Screen Printout Routines

MDFS::Info.Comp.Spectrum.ProgTips.FastScreen

The Spectrum screen is a simple memory-mapped display. The Spectum ROM provides a comprehensive
redirectable screen printout routine via RST &10. However because of this flexibility and functionality, the ROM
code is slow and bulky. If you don't need this functionality it is easy to write a short and fast screen printout
routine.

fastprn1.asm	The simplest character printout routine just copies the character bitmap from a character set pointed to by CHARS to screen, keeping a note of the screen output address in DFCC (Display File Current Character) and incrementing it after each character. Code size: 36 bytes Code speed: 456T per character, +10T if print position goes over a screen third boundary On entry, A=character On exit, AF/BC/DE/HL corrupted No colour setting, uses current screen colours No checking for characters out of range No check for wrapping past the bottom of the screen
fastprn2.asm	A simple calculation converts the character output address to the address of the corresponding attribute location, then the colours can be set for the character just printed. Code size: 49 bytes Code speed: 524T per character, +10T if print position goes over a screen third boundary On entry, A=character On exit, AF/BC/DE/HL corrupted Uses selectable colours No checking for characters out of range No check for wrapping past the bottom of the screen
fastprn3.asm	So far the code just indexes straight into the character definitions without any checking for non-printable characters. Adding a check for character 32 lets control codes be ignored, and a check for characters over 127 lets block graphics and User Defined Characters be used. As there is a bug in the Spectrum's colour commands, if this code is attached to a stream it has to exit with Carry clear when called with control codes. Also, this code treats all characters from 144 to 255 as User Defined Characters pointed to by UDC. There is a subroutine in the ROM that generates the block graphics bitmaps which can be called directly. Otherwise, the code can be duplicated and used inline in the code. Code size: 74 bytes Code speed: 559T per character (CHARS), 996T per character (block graphics), 593T per character (UDC), +10T if print position goes over a screen third boundary On entry, A=character On exit, AF/BC/DE/HL corrupted Uses selectable colours Filters out and ignores control codes Prints block graphics and User Defined Characters No check for wrapping past the bottom of the screen
fastprn4.asm	It is a simple addition is to preserve all the registers so the code doesn't effect the calling code. This is not needed if the code is attached to a Spectrum stream as the stream handler preserves the main register set. Code size: 84 bytes Code speed: 559T per character (CHARS), 996T per character (block graphics), 593T per character (UDC), +10T if print position goes over a screen third boundary +111T if registers preserved On entry, A=character On exit, All registers preserved Uses selectable colours Filters out and ignores control codes Prints block graphics and User Defined Characters No check for wrapping past the bottom of the screen
fastprn5.asm	We've now almost got a complete screen driver. It is simple to add movement control characters which add or subtract 1 or 32 to the current output position, character 13 to move to the start of the next line, and character 12 to clear the screen. These more than doubles the size of the code to implement, though we save some code by using the CHR$9 code to update the print position after printing a character. Code size: 149 bytes Code speed: 592T per character (CHARS), 1031T per character (block graphics), 626T per character (UDC), +2T if print position goes over a screen third boundary +111T if registers preserved On entry, A=character On exit, AF/BC/DE/HL corrupted Uses selectable colours Filters out and ignores control codes Prints block graphics and User Defined Characters No check for wrapping past the bottom of the screen
fastprn6.asm	All the previous code assumes the character output position was on the screen and didn't check if it moved off the top or bottom. A few more bytes adds a simple check which moves the output position back onto the screen if it has moved off. A further improvement would be to scroll the screen when the output position moved off. Code size: 168 bytes Code speed: 635T per character (CHARS), 1075T per character (block graphics), 669T per character (UDC), +2T if print position goes over a screen third boundary, +111T if registers preserved On entry, A=character On exit, AF/BC/DE/HL corrupted Uses selectable colours Implements movement control codes Prints block graphics and User Defined Characters Checks for wrapping past the bottom of the screen
fastprn7.asm	Further improvements would be to implement colour control codes and the AT position control code. This code includes a subroutine to set the character output position. Code size: 200 bytes Code speed: 635T per character (CHARS), 1075T per character (block graphics), 669T per character (UDC), +2T if print position goes over a screen third boundary, +111T if registers preserved On entry, A=character On exit, AF/BC/DE/HL corrupted Uses selectable colours Implements movement control codes Prints block graphics and User Defined Characters Checks for wrapping past the bottom of the screen Routine to set output position On entry, L=X position, H=Y position
prnattr2.asm prnattr7.asm	Instead of storing the current print output position as the address in the screen bitmap memory (DFCC), the method used by Jet Set Willy can be used, storing the address in the attributes memory of the current print output position. The advantage of this is that stepping from one character cell to the next is a simple case of adding one, there is are no steps over screen thirds. Also, converting from attribute address to display address is slightly shorter and faster than the other way around. These two source files are the equivalent of their similar numbered DFCC-based source files, using the attribute memory address as the output address. prnattr2.asm: 41 bytes, 493T per character prnattr7.asm: 189 bytes, 511T/1089T/545T per character

Authored by J.G.Harston
Last update: 12-Jun-2012