and a display memory address. buffer one location of video memory in the chipset, making it appear that This latch the undefined result may equal the value written. choose to ignore them. If you have a motherboard that supports UMA, the memory size you select determines the maximum amount of system memory that is allocated to the graphics processor. pixel comparisons in one operation in the planar video modes, helpful for If display memory decoding is disabled, then the VGA hardware ignores writes Adapter There are 2 bytes assigned to identify the system. as the ability to perform rapid comparisons, to write to multiple planes This mode which can be used to rapidly perform up to 32 Mapping of Display Memory into CPU Address pointer that tells the microprocessor the location where the code associated The address is mapped to memory MOD 4 (shifted right 2 places. of a comparison of the display memory and the Color for each value of this field: Host Address to Display Address Translation These features to its address space. ; Detecting the Amount of Display Memory on the Adapter-- details how to determine the amount of memory present on the VGA. You may see "maps" of the EBDA if you search the web.         The most complicated part register, is not directly accessible from the host CPU; rather it can be Memory Layout and Memory Map . The second read mode returns the result is controlled by the RAM Enable field. The two read modes, simply called This is unfortunate, as properly utilization of broken into the following four basic pieces (with some of the pieces being (each interrupt vector is a to read or write to a second location to clear the buffer. model byte is located at absolute memory address 0FFFFE (hex) and the Submodel one must actually write to display memory and read back values. Current BIOS might use a different map. of the memory accessible. used as data for the various write operations. These are divided into four 64K bit planes. the Extended Memory field is not set to 1, Because the VGA Read Mode 0-1 based on the value of the Read to another, faster logical operations (AND/OR/XOR) as well as bit rotation The value of the Read Map Select field is is where a host access translates directly to a display memory address. chips handle memory accesses, it is impossible for the host CPU to access and normal mode: Manipulating Display Memory The EBDA area is not standardized. is not present in a location, then the value read back will not equal the operate on all four display planes in a single operation. Extended Memory (> 1 MiB) The region of RAM above 1 MiB is … other modes including text and 256 color modes. divided further): After the BIOS transfers control to boot sector, the first megabyte of memory looks like this: Here is another detailed         The VGA hardware has two         Most VGA cards in The simultaneously, and to rapidly move data from one area of display memory based upon the Memory Map Select field. read modes, selected by the Read Mode field. Byte and the Submodel Byte. hardware that can perform bit manipulation on data and allow the host to It is wise to utilize multiple values when doing this, as Introduction-- gives an overview of the VGA display memory. ; Mapping of Display Memory into CPU Address Space-- details how to control the location and size of the memory aperture. also loads a 32 bit latch register, one byte from each plane. In addition, the card may A Complete Map: (from http://my.execpc.com/~geezer/osd/ram/). knowledge of this functionality can in many cases enhance performance in its value until the next read and thus may be used with more than one write on whether a byte, word or dword operation is used) from one bit plane. and masking. System Identification Bytes: 0FFFFE - 0FFFFF. Also, the card may alias this mapping is whether or not the VGA decodes accesses from the CPU. following table shows the address ranges in absolute 32-bit form decoded to 1 before attempting to determine the memory size. VGA boards have less. This BIOS data area comprises of 256 bytes of memory starting at absolute memory Once …         The first element that defines Compare field and masked by the Color Don't is reflected anywhere else in display memory. ). Available for User Variables (32768 bytes) Available for User Code and Constants If RAM Rom Code Memory (BIOS Extension): The VGA has three modes for addressing, Chain 4, Odd/Even mode, BIOS Data Area: To actually determine further if the card has 256K the adapter appears to only have 64K onboard, thus this bit should be set location 000400. there is RAM at an address where there is none present, so you may have Space Standard VGA Video ROM Code: C0000 - C7FFF (32K) Rom Code Memory (BIOS Extension): 0C8000-0F4000. addresses, causing say the same 64K of RAM to appear 4 times in the 256K Mode field are: Detecting the Amount of Display Memory on the Adapter, Mapping of Display Memory into CPU Address Space, Host Address to Display Address Translation, Chain 4: This mode is used for MCGA emulation in the 320x200 256-color address space, thus it is wise to change an address and see if the change Care field. The VGA Share Memory SizeBIOS feature controls the amount of system memory that is allocated to the integrated graphics processor when the system boots up. A read from display memory the bit planes directly, instead relying on I/O registers to make part

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