---
title: OSTestAndSet
framework: kernel
role: symbol
role_heading: Function
path: kernel/1576456-ostestandset
---

# OSTestAndSet

Bit test and set operation, performed atomically with respect to all devices that participate in the coherency architecture of the platform.

## Declaration

```occ
Boolean OSTestAndSet(UInt32 bit, volatile UInt8 *startAddress);
```

## Parameters

- `bit`: The bit number in the range 0 through 7.
- `startAddress`: The address of the byte to update atomically.

## Return Value

Return Value true if the bit was already set, false otherwise.

## Discussion

Discussion This function guarantees atomicity only with main system memory. It is specifically unsuitable for use on noncacheable memory such as that in devices; this function cannot guarantee atomicity, for example, on memory mapped from a PCI device. Additionally, this function incorporates a memory barrier on systems with weakly-ordered memory architectures. The OSTestAndSet function sets a single bit in a byte at a specified address. It returns true if the bit was already set, false otherwise.

## See Also

### Test Operations

- [OSTestAndClear](kernel/1576463-ostestandclear.md)