Advanced Micro Devices - FTP

CHAPTER 3Display Memory Bus3.3 FONT STORAGE IN KANJI ROMSThe Font3.3.1 IntroductionThis note describes a method of storing very large fontsin a Am95C60 Quad Pixel Dataflow Manager (QPDM)system. This method avoids the expense and boardspace that would be required to keep the fonts in VRAMby allowing them to reside in (relatively slow) MOSROMs.3.3.2 String Operations on the QPDMThe QPOM provides a powerfultext manipulation facility,described in detail in Chapter 10 ofthe Technical Manual.An overview is provided here.r- XII = -+I1K 2K 3K 4KA font is stored in display memory (typically with InputBlock) and the QPDM is notified of the location of the fontusing the Set Character Font Base instruction. Each(there may be two) font contains up to 4096 characterentries, all beginning at the same X address. See Figure3.3-1. Each character description begins with a 16-bitattribute word which specifies the size of the character.This is followed by as many pattern words as are requiredto describe the character. In the font described in thissection, each character is 24 pixels by 24 scan lines.With this mechanization, character are separated horizontallyby up to eight blank pixels.1 st Level 2nd LevelVRAM Kanji KanjiONE SCAN LINE---------7 1 /2 scan linesnot usedPID 09862A 3.3-1Figure 3.3·1 Display Memory Layout3·40