FMSS

FMSS is seemingly the name of the flash memory controller on the S5L8702, S5L8710, S5L8720 and S5L8730.

There is no publicly available information about it, and the following has been gathered from reverse engineering RetailOS and iOS builds for the S5L8720 (iPod touch). A very similar controller is present in the S5L8700X datasheet.

Subsystems

FMC (Flash Memory Controller)

This is the component responsible for the actual bus transfers on the NAND bus.

Offset	Register Name	Description
0x000	FMCTRL0	General control register. Bit 0: Enable Bits [1..8]: CE/Bank number Bit 10: DMA enable? Bits [12..14]: Hold clocks Bits [16..18]: Setup clocks Bits [28..30]: EDO clocks
0x004	FMCTRL1	Transfer control register. Bit 0: Start address transfer. Bit 1: Start read transfer. Bit 2: Start write transfer. Bit 4: ??? Bit 5: Clear ??? Bit 6: Clear write FIFO Bit 7: Clear read FIFO
0x008	FMCMD	NAND command number. Eg. 0x90: read NAND ID. Documented in JEDEC docs and NAND chip datasheets.
0x00C	FMADDR0	Lower bits of address to be written in NAND address transfer. See JEDEC docs and NAND chip datasheets.
0x010	FMADDR1	Higher bits of address to be written in NAND address transfer. See JEDEC docs and NAND chip datasheets.
0x02C	FMANUM	Number of bytes to transfer during address transfer minus one (ie. countdown counter).
0x030	FMDNUM	Number of bytes to transfer during data transfer minus one (ie. countdown counter).
0x048	FMSTAT	Controller status. Bit 0: flash busy? Bit 1: command done. Write to clear. Bit 2: address done. Write to clear. Bit 3: transfer done. Write to clear. Bit 23: flash has become busy? Write to clear.

To be documented fully.

ECC (Error Correction Code)

To be documented.

CS (Code Sequencer)

A little custom core that executes a custom bytecode. 9 32-bit general purpose registers. Controlled by the host CPU. It has access to the host memory and the rest of the FMSS peripherals, and operates by performing accesses to the FMC and ECC subsystems.

The bytecode is documented at lemonjesus/S5L8702-FMISS-Tools.

Offset	Register Name	Description
0xC04	CS_IP	Sequencer's instruction pointer.
0xC08	CS_STATUS
0xC0C	CS_IRQ
0xC60	CS_BUF_RST
0xC64	CS_BUF_RST_OK
0xC6C	CS_BUF_START

Other devices / SoCs

S5L8700X (non-Apple) and S5L8900

A similar controller is present, called simply the FMC. It has no code sequencing functionality.

On the S5L8900 the built-in CalmRISC16e core that's part of the ADM (Audio DSP Module) is used as a code sequencer in iOS.