MemoryTypeBits

memoryTypeBits is a bitmask used in Vulkan that describes which memory types on a physical device can be used to back a particular resource, such as a buffer or an image. This mask is returned as part of VkMemoryRequirements by functions like vkGetBufferMemoryRequirements and vkGetImageMemoryRequirements. Each bit in the mask corresponds to a memory type described by VkPhysicalDeviceMemoryProperties. If bit i is set, memory type i is a candidate for allocating memory for that resource, subject to any additional property requirements.

In practice, a memory type is described by a propertyFlags set for each memory type, such as DEVICE_LOCAL, HOST_VISIBLE, HOST_COHERENT, and HOST_CACHED. To allocate memory, an application selects a memory type index i that satisfies two conditions: the i-th bit of memoryTypeBits is set, and the corresponding memory type’s propertyFlags meet the resource’s required properties. If no such i exists, the allocation cannot satisfy the resource’s requirements on that device.

The memoryTypeBits mask is device-specific and may vary across implementations. It does not guarantee an optimal choice by itself; it only identifies which memory types are compatible. Programmers typically scan for a suitable memory type index using memoryTypeBits in combination with the desired propertyFlags, then proceed to vkAllocateMemory with that type index.

Overall, memoryTypeBits is a fundamental part of Vulkan’s memory allocation model, enabling the system to report compatibility between resources and available memory types while allowing applications to select memory with the desired performance characteristics.