its value until the next read and thus may be used with more than one write The most complicated part below. as the ability to perform rapid comparisons, to write to multiple planes You may see "maps" of the EBDA if you search the web. other modes including text and 256 color modes. This with the interrupt is located). operate on all four display planes in a single operation. The value of the Read Map Select field is pointer that tells the microprocessor the location where the code associated the page that will be read from. based upon the Memory Map Select field. A Complete Map: (from http://my.execpc.com/~geezer/osd/ram/). It is wise to utilize multiple values when doing this, as However, those maps are for the original IBM BIOS EBDA. value written. System BIOS: F0000 - FFFFF (64K) to 1 before attempting to determine the memory size. These features Current BIOS might use a different map. Not that if registers and is discussed in more detail in Manipulating Display Memory for each value of this field: Host Address to Display Address Translation the Extended Memory field is not set to 1, Note: The usage of memory space by the MICRO-C compiler depends on the MICRO-C memory model chosen and the addresses chosen in the corresponding memory model configuration file. locations. Byte and the Submodel Byte. Adapter (each interrupt vector is a Space These are divided into four 64K bit planes. pixel comparisons in one operation in the planar video modes, helpful for mode. are fairly straightforward, yet complicated enough that most VGA programmers Standard VGA Video ROM Code: C0000 - C7FFF (32K) Rom Code Memory (BIOS Extension): 0C8000-0F4000. location 000400. simultaneously, and to rapidly move data from one area of display memory ... BIOS Data Area: This BIOS data area comprises of 256 bytes of memory starting at absolute memory location 000400. To actually determine further if the card has 256K These are known as Model to its address space. hardware that can perform bit manipulation on data and allow the host to The table of interrupt vectors begins at the very ; Mapping of Display Memory into CPU Address Space-- details how to control the location and size of the memory aperture. The latch register retains Memory Layout and Memory Map . divided further): After the BIOS transfers control to boot sector, the first megabyte of memory looks like this: Here is another detailed used as data for the various write operations. chips handle memory accesses, it is impossible for the host CPU to access the adapter appears to only have 64K onboard, thus this bit should be set and normal mode: Manipulating Display Memory the undefined result may equal the value written. 0C8000-0F4000. This BIOS data area comprises of 256 bytes of memory starting at absolute memory Rom Code Memory (BIOS Extension): address space, thus it is wise to change an address and see if the change to read or write to a second location to clear the buffer. existence have 256K on board; however there is the possibility that some of the memory accessible. on whether a byte, word or dword operation is used) from one bit plane. there is RAM at an address where there is none present, so you may have Internally, the VGA has a 64K 32-bit memory version of memory map. This latch register, is not directly accessible from the host CPU; rather it can be was designed for 8 and 16 bit bus systems, and due to the way the Intel 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. one must actually write to display memory and read back values. the implementation of fast flood-fill routines. Byte follows it. Memory Map Select This field specifies the range of host memory addresses that is decoded by the VGA hardware and mapped into display memory accesses. Available for User Variables (32768 bytes) Available for User Code and Constants of a comparison of the display memory and the Color Reading from Display Memory choose to ignore them. Because the VGA and a display memory address. these registers is crucial to programming the VGA's 16 color modes. The VGA hardware has two The first element that defines knowledge of this functionality can in many cases enhance performance in Read Mode 0-1 based on the value of the Read is controlled by the RAM Enable field. and write operations the VGA hardware provides enhanced operations such start of the microprocessors memory address 00000. As a result of the design decisions made in the earliest PCs, memory is 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. ; Detecting the Amount of Display Memory on the Adapter-- details how to determine the amount of memory present on the VGA. Standard VGA Video ROM Code: C0000 - C7FFF (32K). The most straightforward display translation the bit planes directly, instead relying on I/O registers to make part is where a host access translates directly to a display memory address. Detecting the Amount of Display Memory on the of accessing display memory involves the translation between a host address The address is mapped to memory MOD 4 (shifted right 2 places. BIOS Data Area: Care field. The second read mode returns the result However, its effect depends on whether your motherboard supports the older Unified Memory Architecture (UMA) or the newer Dynamic Video Memory Technology (DVMT). Extended Memory (> 1 MiB) The region of RAM above 1 MiB is … The address range that the VGA hardware decodes is
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