Porting Unix to the 386: Designing the Software Specification

Physical Memory Map The ISA bus physical memory layout is outlined in Figure 9. The memory is broken into three parts: base memory, I/O device memory, and extended memory. RAM is split up on this standard, with a base memory section, holding up to 640 Kbyte of memory, starting at address 0 and ending at the beginning of device memory. Remaining memory is located starting at address 0x100000 (above 1 Mbyte) and extending to as much as 0xFFFFFF (16 Mbytes).

Physical
Memory
Address

Max. Memory

0x100000

0x0A0000

0x000000

Extended Memory

Device Memory

Base Memory

Figure 9 - ISA Physical Memory Map

Between the base and extended RAM regions lies device memory, where display adapter cards and LAN cards use special RAM buffers. This region, called the "hole," is a nuisance for UNIX ports, because we would rather see contiguous memory. Although we do have a means of making memory appear contiguous through the use of virtual memory, this does us no good when we must work with physical memory addresses during system bootstrap, hardware DMA devices, and physical memory allocation structures.

If extended memory is not available, we must temporarily reside in the MS-DOS 640-Kbyte base-memory dungeon. This is truly hell for memory-consumptive UNIX systems. Fortunately, this occurs only when the system is "misconfigured" during the configuration or boot processes, and is not a "normal" situation.

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