Only one session can be open per service at a time. If a session is already open for a service, MCU module will wait for the thread handling the session to terminate(triggered by the session being closed by the user process), then it accepts the new session. The commands for each service are handled by separate threads.

While before these functions are handled, the MCU interrupt with bitmask 0x800 is enabled, then after the commands were handled the MCU interrupt bits 0x1800 get cleared.

Beg the sysmodule to return the datetime registers in decimal form instead of as a Binary Coded Decimal

The Old3DS/New3DS MCU sysmodules are identical except that the MCU firmware binary written via I2C is different. The size of that binary is the same. The only different words in .text are for the version of that MCU fw binary.

These reside in mcu sysmodule .rodata, are uploaded to MCU register 0x05 and are usually 0x4003 bytes in size (the actual firmware is 0x4000 bytes preceeded by a 3 byte magic header “jhl” which switches the I2C comms into flash write mode). Switching requires register 0x05 (at address 0xFFBA9) to contain 0x6A (’j’), register 0x06 containing 0x68 (’h’), and writing 0x6C (’l’) to register 0x07. The actual flashing sequence is only signaled (code at 0x3312-0x331A) when writing register 0x07, it’s skipped otherwise. Register 0x07 gets written anyways, just the actual signaling is skipped if the conditions aren’t met.

Before the upload could commence, external MCU interrupts are turned off via GPIO command 0x00020080(0, 0x40000), then after the upload completed, the sysmodule waits exactly one second for the MCU to reboot, then turns external MCU interrupts back on via gpio:MCU command 0x00020080(0x40000, 0x40000).

There exists an alternate code path in very old MCU_FIRM versions where uploading is done using register 0x3B (if register 0x0F is zero and 0x10 is 1). Register 0x3B is part of the RTC alarm registers on recent versions of MCU.

On dev-units, the user-facing representation of this firmware version is displayed by first subtracting 0x10 from the major field (raw register 0x00). It is these user-facing versions that are displayed in the table below. It is unknown what bit4 (0x10) actually represents, but it is seemingly always set.