Memory Space Basics

19.1 Memory Space Basics

Simics memory-spaces are handled by the generic memory-space class. A memory-space object implements interface functions for memory accesses, and it has attributes specifying how the mappings are setup.

The most important attribute in a memory-space is the map attribute. This is a list of mapped objects that may contain devices, RAM and ROM objects, and other memory-spaces. In addition to the map attribute, there is also a default_target attribute that is used for accesses that don't match any of the targets in the map list.

Python example of a memory-space object where already created objects are mapped into a new memory space:

@mem = pre_conf_object('phys_mem', 'memory-space')
@mem.map = [[0x00000000, conf.ram0,    0, 0,          0xa0000],
            [0x000a0000, conf.vga0,    1, 0,          0x20000],
            [0x000c0000, conf.rom0,    0, 0,          0x10000],
            [0x000f0000, conf.rom0,    0, 0,          0x10000],
            [0x00100000, conf.ram0,    0, 0x100000, 0xff00000],
            [0xfee00000, conf.apic0,   0, 0,           0x4000],
            [0xffe81000, conf.hfs0,    0, 0,               16],
            [0xffff0000, conf.rom0,    0, 0,          0x10000]]
@mem.default_target = [conf.pci_mem0, 0, 0, conf.pci_mem0]
@SIM_add_configuration([mem], None)

The fields for an entry in the map list are as follows:

base: The start address of the mapping in the memory space.
object: Reference to the mapped object.
function: An object specific identification number for this mapping. The function number is typically used by objects that have several mappings. When an object is accessed, it can use the function number to figure out what mapping it was accessed through.
offset: The start offset in the target object. This is often used when a memory-space is mapped in another memory-space. Example: memory-space B is mapped in space A at base 0x4000 with length 0x1000, and with offset 0x2000. If an access is made in space A at address 0x4010, it will be forwarded to space B at address 0x2010. Without any offset in the mapping, the resulting address would have been 0x10.
length: The size of the mapping in bytes.
target: (optional) If object isn't the final destination for the access, then target is a reference to the actual target object. This is described in more details in the section about different mapping types.
priority: (optional) The priority of the mapping. Default is 0 (highest priority), and the lowest is 256. If mappings overlap, then the priority field specified what mapping that has precedence. It is an error if overlapping mappings have the same priority. Usually overlapping mappings should be avoided, but for bridges that catch unclaimed accesses in specific address ranges the priority field is useful. There are also devices that have overlapping mappings that have different priorities, and in that case the priority field in the map list can be used.
align-size: (optional) The align-size can be be used if a target does not support large accesses. Accesses that crosses an alignment boundary will be split into several transactions by the Simics memory system. By default this will be set to 4 bytes for port space devices, 8 bytes for other devices and 4096 or 8192 for memory.
reverse-endian: (optional) Some device mappings reverse the byte order of data, to support mixed-endian environments. The reverse-endian field can be used to model this behavior. It will reverse the byte-order of data for mappings that have an align-size of 2, 4 or 8 bytes.

Note: In Simics versions prior to 2.0, the priority field was a "reverse-endian" flag. Old checkpoints will automatically be updated with this field cleared, since the handling of endian swapping has changed. This old "reverse-endian" flag was obsoleted since devices in 2.0 read and write data with explicit endianness, and reversing the byte-order of all accesses in a memory-space often was incorrect.

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