Designing around a 6502 (was Re: Assembly on a Apple IIc+)

Virtual addressing always includes address translation. And Memory protection is again not necersary part of a MMU, and also not needed for multitasking nor virtual addressing. It's a nice feature, but so is reentrant coding and a hardware stack. The basics for virtual memory in a real systems is a way to translate a programm address (virtual ones) into a physical (*1) memory address, a way to detect access to a not allocated address and a way to restart the access issueing instruction. For translation a simple unit consisting of a RAM whose address is taken from some bits of the CPU (virtual) address, and the data output is connected to the physical address lines. The number of lines taken to address the translation RAM defines the block size, while the nuber of data bits used as replacement (times block size) defines the maximum physical address space - which may larger or smaller than virtual (*1) Now we have already virtual addressing - in this way every address desoding is already a form of virtual addressing, since you assign CPU addresses to physical memory addresses. Next step is detecting address faults. This can be done either by a seperate on bit wide RAM with a tag to define for every virual address block the state (valid/not valid), or by defineing a unique value which is used as not assigned. A usefull combination could be all bits high, so all we need is a NAND gate to produce a /invalid signal as soon as the CPU accesses a not allocated page. In either way the result is used to generate an interupt to tell the CPU about the address violation. Last but not least the interupt function has to compute the faulting page, see what's necersary here (depending on OS and allocation status either task termination or loading the page from some swap device int physical memory and modifying th translation table), and if needed analyzeing the last instruction and restarting it. I haven't seen anything about memory protection in here and I haven't seen anything about multitasking either. Gruss H. (*1) Often the term real memory is used, but that's not correct. Physical memory is the memory installed in a machine, while real address space is the addressable amount of RAM by a CPU without using virtual addressing. Virtual address space can never excede real address space (After all, it's the maximum address range generated by the (logical) CPU), while physical memory can go beyond real or virual address space. (*2) Most people thinks about virtual addressing as a way to extend available memory beyond physical memory. A scheme used by simple unix systems, and early mainframes. More often than not, virtual addressing is used to extend the CPU address space to handle a larger REAL memory than the CPU could manage otherwise. Already since the early 80s, IBM compatible mainframes allow real memory to go way beyond the 24 (or later 31 Bit) address space of the (user visible) CPU. For example we had did use in 1984 a CPU with 64 meg of RAM while the CPU address space was only 16 meg (only 12 meg user address space). Before the new 64 bit extension, actual mainframes often had a physical memory of more than 100 GByte, but only a CPU address space of up to 2 gig (31 Bit addressing). The 64 Bit extension offered a way to manipulate a larger address space on user level. Except for very early systems, where memory was extreme expensive, this was also the same for 8 Bit CPUs. Physical memory did exeed the address space of the CPU. So memory management for 8-Biters was a way to manage more than 64K for a CPU which could only address 64K. Things like pageing, or page faults where never an issue. I guess everyone remembers CPM 3.0 and it's management scheme. Virtual addressing without pageing or multi tasking. -- VCF Europa 4.0 am 03./04. Mai 2003 in Muenchen http://www.vcfe.org/