The Memory Mapped I/O Range in which I/O devices can reside is further divided into subregions: This portion of the memory map was defined when 1 MB of memory was considered very large and has been retained for platform compatibility. The IA-32 memory map from zero to 1 MB is built from a mix of system memory and MMIO. A register in the SOC called TOLM indicates the top of local memory-you can assume that the DRAM is mapped from 1 MB to TOLM. The two address ranges are known as the Main Memory Address Range and the Memory Mapped I/O (MMIO) Range. The first is the address range that when decoded accesses the DRAM, and the second is a range of addresses that are decoded to select I/O devices. The address map within the memory address space on Intel systems (and in fact in most systems) is split into two separate sub ranges. This decision logic might match the address to that of the system DRAM controller, which then generates a transaction to the memory devices the transaction could be routed to a hardware register in an Ethernet network controller on a PCI bus to indicate that a packet is ready for transmission. When the processor generates a read or write, the address is decoded by the system memory address decoders and is eventually routed to the appropriate physical device to complete the transaction. Many other embedded processor architectures such as ARM or PowerPC have only a memory address space.
#Increase memory address space for mac software
Since accesses to I/O devices through I/O space are relatively time consuming, most cases avoid use of the I/O space except for supporting legacy features (Intel platform architecture features that have been retained through the many generations of processors to ensure software compatibility). The I/O space is far smaller (only 64 kB) and can only be accessed via IN/OUT instructions. Access to memory space is achieved by memory read/write instructions such as MOV. For example, on a 32-bit system the memory space ranges from 0 to 4 GB, although not all addresses in this range map to a device or memory.
It occupies the entire physical address space of the processor. The memory space is actually the primary address space and it covers the DRAM and most I/O devices. IA-32-based platforms have two distinct address spaces, memory space and input/output space. It’s important to note that there can be different points of view from different agents in the system in particular, on some embedded devices the memory map is different when viewed from I/O devices or from devices that are attached on an I/O bus, although this is not the case on Intel platforms. The system memory map is generated from the point of view of the processor. The memory map is a list of physical addresses of all the resources on the platform, such as the DRAM memory, the interrupt controllers, and I/O devices. Peter Barry, Patrick Crowley, in Modern Embedded Computing, 2012 System Memory MapĪ key to understanding any embedded system starts with a thorough understanding of the memory map. It also allows us to specify the address in the out (N),a instruction in just 1 byte, instead of the 2 bytes that would have to be provided if we used a 16-bit port address.Īssume there are three output ports in the computer, at port addresses 00, 01, and 02. We let port addresses be just 8 bits since this allows 256 input ports and 256 output ports – more than enough. This allows ports to be I/O-mapped rather than memory-mapped. In concept, we now have two address spaces, one for memory and another for ports. Note that, just as the MREQ signal indicates that the address bus is carrying a memory address, the IORQ signal indicates that the address bus is carrying a port address. Quite simply, we let the G80 assert a new signal, IORQ, when it has placed a port address on the address bus, and we add to the G80 instruction set an instruction out (port_address),a which transfers data from register A to the port having port address port_address. To overcome this inconvenience, we provide the G80 with an alternative method of addressing ports, a method used in many other microprocessors. This is particularly inconvenient where all the available memory address space is likely to be used by memory devices. Using the memory address space for the port limits the use of that space for memory. Wilson, in Embedded Systems and Computer Architecture, 2002 9.2 Port address space
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