http://www.opengl.org/registry/specs/NV/shader_buffer_load.txt

Overview    At a very coarse level, GL has evolved in a way that allows     applications to replace many of the original state machine variables     with blocks of user-defined data. For example, the current vertex     state has been augmented by vertex buffer objects, fixed-function     shading state and parameters have been replaced by shaders/programs     and constant buffers, etc.. Applications switch between coarse sets     of state by binding objects to the context or to other container     objects (e.g. vertex array objects) instead of manipulating state     variables of the context. In terms of the number of GL commands     required to draw an object, modern applications are orders of     magnitude more efficient than legacy applications, but this explosion     of objects bound to other objects has led to a new bottleneck -     pointer chasing and CPU L2 cache misses in the driver, and general     L2 cache pollution.    This extension provides a mechanism to read from a flat, 64-bit GPU     address space from programs/shaders, to query GPU addresses of buffer    objects at the API level, and to bind buffer objects to the context in    such a way that they can be accessed via their GPU addresses in any     shader stage.         The intent is that applications can avoid re-binding buffer objects     or updating constants between each Draw call and instead simply use     a VertexAttrib (or TexCoord, or InstanceID, or...) to "point" to the     new object‘s state. In this way, one of the cheapest "state" updates     (from the CPU‘s point of view) can be used to effect a significant     state change in the shader similarly to how a pointer change may on     the CPU. At the same time, this relieves the limits on how many     buffer objects can be accessed at once by shaders, and allows these     buffer object accesses to be exposed as C-style pointer dereferences    in the shading language.    As a very simple example, imagine packing a group of similar objects‘     constants into a single buffer object and pointing your program    at object <i> by setting "glVertexAttribI1iEXT(attrLoc, i);"    and using a shader as such:        struct MyObjectType {            mat4x4 modelView;            vec4 materialPropertyX;            // etc.        };        uniform MyObjectType *allObjects;        in int objectID; // bound to attrLoc                ...        mat4x4 thisObjectsMatrix = allObjects[objectID].modelView;        // do transform, shading, etc.    This is beneficial in much the same way that texture arrays allow     choosing between similar, but independent, texture maps with a single    coordinate identifying which slice of the texture to use. It also    resembles instancing, where a lightweight change (incrementing the     instance ID) can be used to generate a different and interesting     result, but with additional flexibility over instancing because the     values are app-controlled and not a single incrementing counter.        Dependent pointer fetches are allowed, so more complex scene graph     structures can be built into buffer objects providing significant new     flexibility in the use of shaders. Another simple example, showing     something you can‘t do with existing functionality, is to do dependent    fetches into many buffer objects:        GenBuffers(N, dataBuffers);        GenBuffers(1, &pointerBuffer);        GLuint64EXT gpuAddrs[N];        for (i = 0; i < N; ++i) {            BindBuffer(target, dataBuffers[i]);            BufferData(target, size[i], myData[i], STATIC_DRAW);                        // get the address of this buffer and make it resident.            GetBufferParameterui64vNV(target, BUFFER_GPU_ADDRESS,                                       gpuaddrs[i]);             MakeBufferResidentNV(target, READ_ONLY);        }        GLuint64EXT pointerBufferAddr;        BindBuffer(target, pointerBuffer);        BufferData(target, sizeof(GLuint64EXT)*N, gpuAddrs, STATIC_DRAW);        GetBufferParameterui64vNV(target, BUFFER_GPU_ADDRESS,                                   &pointerBufferAddr);         MakeBufferResidentNV(target, READ_ONLY);        // now in the shader, we can use a double indirection        vec4 **ptrToBuffers = pointerBufferAddr;        vec4 *ptrToBufferI = ptrToBuffers[i];    This allows simultaneous access to more buffers than     EXT_bindable_uniform (MAX_VERTEX_BINDABLE_UNIFORMS, etc.) and each    can be larger than MAX_BINDABLE_UNIFORM_SIZE.