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Tutorial 04 - Mailboxes
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=======================
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Before we could go on with UART0, we need mailboxes. So in this tutorial we introduce the mailbox interface.
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We'll use it to query the board's serial number and print that out on UART1.
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NOTE: qemu does not redirect UART1 to terminal by default, only UART0!
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Uart.h, uart.c
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--------------
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`uart_hex(d)` prints out a binary value in hexadecimal format.
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Mbox.h, mbox.c
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--------------
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The mailbox interface. First we fill up the message in `mbox` array, then we call
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`mbox_call(ch)` to pass it to the GPU, specifying the mailbox channel.
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In this example we have used the [property channel](https://github.com/raspberrypi/firmware/wiki/Mailbox-property-interface),
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which requires the message to be formatted as:
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```
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0. size of the message in bytes, (x+1)*4
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1. MBOX_REQUEST magic value, indicates request message
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2-x. tags
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x+1. MBOX_TAG_LAST magic value, indicates no more tags
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```
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Where each tag looks like:
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```
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n+0. tag identifier
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n+1. value buffer size in bytes
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n+2. must be zero
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n+3. optional value buffer
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```
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Main
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----
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We query the board's serial number and then we display it on the serial console.
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