Книга: Real-Time Concepts for Embedded Systems

3.2.1 Embedded Loader

3.2.1 Embedded Loader

A common approach taken at the early development phase is to write a loader program for the target side, which is called the loader, and use the loader to download the image from the host system. In the scenario shown in Figure 3.1, the loader has a small memory footprint, so it typically can be programmed into a ROM chip. A data transfer utility resides on the host system side. The loader works in conjunction with its host utility counterpart to perform the image transfer.

After the loader is written, it is programmed into the ROM. Part of the same ROM chip is occupied by the boot image. At a minimum, this boot image (typically written by a hardware engineer) consists of the code that executes on system power up. This code initializes the target hardware, such as the memory system and the physical RAM, into a known state. In other words, the boot image prepares the system to execute the loader. The loader begins execution after this boot image completes the necessary initialization work.

For this transfer method to work, a data transfer protocol, as well as the communication parameters, must be agreed upon between the host utility and the target loader. The data transfer protocol refers to the transfer rules. For example, a transfer protocol might be that the image transfer request should be initiated from the loader to the host utility; in which case, the host utility sends out the image file size followed by the actual image, and the loader sends an acknowledgement to the host utility upon completion. Data transfer rate, such as the baud rate for the serial connection, and per packet size are examples of communication parameters. The loader and the utility program operate as a unit, which is often capable of using more than one type of connection. At a minimum, the transfer takes place over the serial connection. More sophisticated loaders can download images over the network, for example, over the Ethernet using protocols such as the Trivial File Transfer Protocol (TFTP) or the File Transfer Protocol (FTP). In this case, the host utility program is either the TFTP server or the FTP server respectively.

Both proprietary and well-known transfer protocols can be applied in either the serial or the network connection, but more commonly proprietary protocols are used with a serial connection.

The loader downloads the image directly into the RAM memory. The loader needs to understand the object file format (for example, the ELF format) because, as discussed in Chapter 2, the object file contains information such as the load address, which the loader uses for section placement.

The loader transfers control to the downloaded image after the transfer completes. A loader with flash programming capability can also transfer the image into the flash memory. In that case, the board jumpers must be set appropriately so that the processor executes out of flash memory after the image download completes.

A loader can be part of the final application program, and it can perform other functions in addition to downloading images, as discussed in more detail later in this chapter.

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