The ‘mmap()’ methodPurpose of ‘mmap()’Mapping vram to userspace‘mmap()’ syntaxThe ‘page-protection’ bitsThe ‘flags’ bitsThe region to be mappedKernel data-structuresThe ‘munmap()’ syntaxDemo-program: ‘vrammdraw’In-class exercise #1In-class exercise #2The ‘mmap()’ methodAdding the ‘mmap()’ capability to our ‘vram.c’ device-driverPurpose of ‘mmap()’•The ‘mmap()’ system-call creates a new region in the task’s virtual memory map, and associates that region with a file (or with a device-file such as ‘/dev/vram’)•It returns a pointer to this memory area•Thus it allows the program to access the file as if it were an array in user-space•Avoids ‘read()’ and ‘write()’ system-callsMapping vram to userspace virtualaddress-space physicaladdress-spaceHIGHMEMZONENORMALKernelText/DataVRAMVRAMKernelspace UserspaceSTACKTEXT/DATARUNTIMELIBRARIESm map()‘mmap()’ syntax void *mmap( void *start, size_t length, int prot,int flags,int fd,off_t offset );The ‘page-protection’ bits•PROT_READ•PROT_WRITE•PROT_EXEC•PROT_NONEThe ‘flags’ bits•MAP_SHARED•MAP_PRIVATE•MAP_FIXED•MAP_ANONYMOUS•MAP_GROWSDOWN•MAP_DENYWRITE•MAP_EXECUTABLE•MAP_LOCKED•MAP_NORESERVE•MAP_POPULATE•MAP_NONBLOCKThe region to be mappedoffset start(virtual address)file region to be mappedlengthTotal extent of the fileKernel data-structurestask_structmmmm_structpagedirectory pgd_tpgdvmavm_start vm_endvm_area_structvm_start vm_endvm_start vm_endvm_start vm_endvm_start vm_endvm_start vm_endvm_start vm_endThe ‘munmap()’ syntax int munmap( void *start, int length );Demo-program: ‘vrammdraw’•We added ‘my_mmap()’ to our ‘vram.c’ device-driver (calling it ‘vramm.c’)•We wrote an application (actually a new version of our ‘vramdraw.cpp’ program) that calls our driver’s ‘mmap()’ method•You can try it out – it’s on class websiteIn-class exercise #1•Write a new version of our ‘grabber1.cpp’ and ‘grabber2.cpp’ demo-programs which will ‘grab’ a screen-image and write it out to a file, but will NOT make system-calls to ‘read()’ from the ‘/dev/vram’ device-file (it will instead use ‘mmap()’ to map the video display memory into user-space so it can be accessed just like an ordinary array)In-class exercise #2•Use the Linux ‘time’ command to compare the execution-speed of your ‘grabber3’ to observed speeds of ‘grabber1’,
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