Advanced Micro Devices - FTP

CHAPTER 3Display Memory Busthe QPOM is not part ofthe address going into the ROMs,but is used to determine which half of the charactershould be fetched.When the QPOM executes a memory read cycle to fetchthe first word ofthe second scan line, X~ will beaone. ThePAL device makes EN_LEFT_WORu' active and theprocess continues.3.3.5 RemarksThe X11 ProblemThis solution requires that the QPOM emit address X"which is not the case for memory configurations 0 and E(see Chapter 12 of Tech Manual). The third majorrevision of silicon (REV. C) supports these two addressbits in configurations 0 and E.An interim solution is diagramed in Figure 3.3-5. TheQPOM is actually programmed for memory configurationC (see Section 12 of the technical manual) so thataddress X" is available. Then, we externally put amultiplexer in front of A., to the VRAMs to make theaddresses the same as they would have been in configurationE. The implementer must insure that no RAMtiming parameters are violated. Row address set-up andhold times should be checked especially carefully.The Form of the Attribute WordThe attribute word should be as follows:BITVALUE15 I 14 I 13 I 121 I 1 I 0 I 011 I 10 I 9 I 8o I 0 I 0 I 07 I 6 5 I 4 I 3 I 2 I 1 I 0o I 0 0 I 1 I 1 I 1 I 0 I 0FIELDHS0 ICOHis setto 1100 (decimal 12). Assuming cell scale is 2, this corresponds to 24 scan lines of active characters. If thecell scale is set to 4, this should be 0110 (decimal 6).S is set to 0000 because we do not want any space above characters.o is set to 00 to force left-to-right character positioning.ICO is set to 01 1100 (decimal 28). Since the character occupies 24 pixels, this leaves four pixels betweencharacters. These pixels are forced to zero by U11. Since eight zeroes are provided, the ICO could be as muchas 32, leaving eight pixels between characters.Attribute SI..O Left SI..O Right SL, Loft SLI RightRASCASCLK-CHARClK·SCAN·LINEEN-ATTRIB-WORD"EN-lEFT·WORD"EN·RIGHT·WORD"PROM 0AddressX·Add",ss 0from QPDM2 3 4Figure 3.3-3 Timing Diagram3-44