Compatibility issues with A20 address lines

Tech 2023-05-17 12:31:51 Source: Network

In early computers, there were only 20 address buses, each of which could address 1MB of memory space. In some newer computers, the address bus has grown to 32, allowing for addressing more memory

In early computers, there were only 20 address buses, each of which could address 1MB of memory space. In some newer computers, the address bus has grown to 32, allowing for addressing more memory. However, in order to be compatible with older software, some modern computers still turn off the A20 address bus during startup in order to access older memory addresses. Turning on the A20 address bus means that the computer will be able to address more memory.

In real mode, memory access takes the form of "segment base address: intra segment offset address", where the segment base address is multiplied by 16 and then the intra segment offset address is added. In real mode, registers are all 16 bits. If both the segment base address and the offset address within the segment are the maximum values of 16 bits, which is 0xffff: 0xffff, the maximum address is oxffff0+0xffff, which is OxlOFF. Due to the address line being 20 bits in real mode, the maximum address space is 1MB, which is 0x00000~0xfffff. The part exceeding 1MB of memory is logically normal, but there is no corresponding part in physical memory. In order to make the "segment base address: intra segment offset address" strategy continue to be available, the CPU adopts the approach of automatically wrapping the part exceeding 1MB to the 0 address, and continuing to map from the 0 address is equivalent to moduling the address to 1MB. The excess memory exceeding 1MB is called the high-end memory area HMA.

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