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The HAL device APIs are modelled around the C APCS, and furthermore can be implemented either in a HAL (with SB) or RISC OS module (with R12). Therefore, exporting SB or requiring its use as a function argument precludes implementing the device in a normal module.
For examples of how to keep track of SB for the HAL case, see AudC or BMU or CPUClk or DMA or GPIO or IDE or Mixer or RTC or SDHCI or CacheC devices, which typically keep it in their HAL workspace referenced relative to the device pointer passed in a1/R0.