Internals

Key concepts

• Go applications built with TamaGo run on bare metal, without any underlying OS. All required support is provided by the Go runtime and driver packages, also written in Go.

• When hardware initialization is provided (like in the imx6ul package), the compiled binaries can be directly executed by the processor, without any need for a 3rd party bootloader. This completely eliminates any non-Go dependency.

Go distribution changes

• TamaGo modifications to the original Go distribution are minimal to ease maintainability and potential upstream acceptance.

• TamaGo support is implemented with a clean separation from other OSes, all modifications are conditional to compilation option GOOS=tamago and do not affect other architectures.

Go runtime changes

• Board support packages are required to define the following functions to support the runtime:

  • hwinit for preliminary hardware initialization
  • printk to provide standard output (typically on a serial console)
  • initRNG and getRandomData to initialize and provide random number generation
  • nanotime1
  • ramStart, ramSize and ramStackOffset to provide RAM layout

• The memory allocation is identical to plan9 with the exception of the brk call which is not required.

• There is no support for threads, just like for Go js/wasm support, and locking is identical to js/wasm.

• The only supported, and required, syscall is write which allows only stdout and stderr file descriptors.

• The crypto/rand support is delegated to the supporting SoC driver, for example with the imx6ul packages support for the RNGB True Random Number Generator.

• The file system support is identical to NaCl and provides a volatile in-memory emulated disk, the file descriptor support is therefore also identical to NaCl to support it.

• Network I/O through Go own net package requires the application to set an external Socket function, see imx-enet or imx-usbnet for examples.

• The testing environment for ARM and RISCV64 architectures runs under Linux natively or using qemu-system-arm and qemu-system-riscv64 via binfmt_misc respectively.

Go application limitations

The GOOS=tamago target maintains full language and standard library support, see the following page for all compatibility notes:

• Compatibility

Default memory layout

  +----------------------------------+ 0000 0000
  |                                  |
  |                                  |
  |                                  |
  |                                  |
  |                                  |
  +----------------------------------+ runtime.ramStart (default for runtime.vecTableStart)
  |                                  |
  |  EXCEPTION VECTOR TABLE (16 kB)  |
  |                                  |
  +----------------------------------+ runtime.ramStart + 0x4000 (16 kB)
  |                                  |
  |        L1 PAGE TABLE (16 kB)     |
  |                                  |
  +----------------------------------+ runtime.ramStart + 0x8000 (32 kB)
  |                                  |
  |       EXCEPTION STACK (16 kB)    |
  |                                  |
  +----------------------------------+ runtime.ramStart + 0xC000 (48 kB)
  |                                  |
  |       L2 PAGE TABLE (16 kB)      |
  |                                  |
  +----------------------------------+ runtime.ramStart + 0x10000 (64 kB)
  |                                  |
  | .text                            |
  |                                  |
  | .noptrdata                       |
  |                Go application    |
  | .data                            |
  |                                  |
  | .bss                             |
  |                                  |
  | .noptrbss                        |
  |                                  |
  +----------------------------------+
  |                                  |
  |                                  |
  |                                  |
  |                                  |
  |              HEAP                |
  |                                  |
  |                                  |
  |                                  |
  |                                  |
  +----------------------------------+ runtime.g0.stack.lo (runtime.go.stack.hi - 0x10000)
  |                                  |
  |                                  |
  |           STACK (64 kB)          |
  |                                  |
  |                                  |
  +----------------------------------+ runtime.go.stack.hi (runtime.ramStart + runtime.ramSize - runtime.ramStackOffset)
  |                                  |
  |             UNUSED               |
  |                                  |
  +----------------------------------+ runtime.ramStart + runtime.ramSize
  |                                  |
  |                                  |
  +----------------------------------+ FFFF FFFF

Additionally the imx6ul package uses the 128KB On-Chip RAM (also known as iRAM) for DMA transfers.

The Go runtime is not direcly aware of this memory area, to prevent any GC operation on it and allow its use for DMA transfers with hardware peripherals.

  +----------------------------------+ mem.dmaStart
  |                                  |
  |           DMA BUFFERS            |
  |                                  |
  +----------------------------------+ mem.dmaStart + mem.dmaSize

Board packages and applications are free to override ramStart and ramSize as well as re-initialize DMA buffers if more space is required.

Interrupts (ARM)

On the ARM architecture interrupts are supported with the following helpers:

• Package gic, which provides a driver for the ARM Generic Interrupt Controller.

• Function ServiceInterrupts, which allows registration of a gorouting as IRQ handler.

• Peripheral drivers functions to enable specific controller IRQs as needed, such as NXP ENET Ethernet driver.

The following projects provide an example use of such helpers:

• Ethernet and USB interrupt handling in tamago-example.

• TrustZone Watchdog enabling, and servicing in GoTEE-example.