Memory-mapped I/O

HarvOS uses a compact MMIO region at 0xffff0000. The CPU accesses it through normal load/store instructions; MPU and SoC address decode decide which block responds.

MMIO map

Base	Block	Use
`0xffff0000`	Debug / kernel model	Debug PC, trap cause, entropy, boot status and DMA status.
`0xffff0030`	IMEM loader	Controlled code load into IMEM and lock.
`0xffff0040`	HNET	Packet-DMA network device for HarvTTP and HarvSQL.
`0xffff0080`	USB host	USB host controller, DMA and PHY/port control.
`0xffff00c0`	HPS block	MiSTer/HPS-compatible access to two block images.

IMEM loader

Offset	Name	Description
`0x00`	`ID`	Identification/status read path.
`0x04`	`CTRL`	Bits: `WRITE`, `LOCK`, `CLEAR`.
`0x08`	`ADDR`	Target IMEM address.
`0x0c`	`DATA`	32-bit instruction word.

HNET packet-DMA NIC

Register	Offset	Description
`CTRL`	`0x04`	`ENABLE`, `LOOPBACK`, `CLEAR`, `PROMISC`, `SIM_LINK`.
`STATUS`	`0x08`	`LINK`, `BUSY`, `DONE`, `FAULT`, `IRQ`, `RX_OP`, `TX_READY`, `RX_VALID`, `RX_ERROR`.
`IRQ_EN`/`IRQ_STAT`	`0x0c`/`0x10`	RX_DONE, TX_DONE, FAULT and LINK events.
`RX_ADDR`/`RX_LEN`	`0x14`/`0x18`	DMA target for incoming frames.
`TX_ADDR`/`TX_LEN`	`0x20`/`0x24`	DMA source for outgoing frames.
`MAC_LO`/`MAC_HI`	`0x28`/`0x2c`	Virtual MAC address.
`FRAME_LEN`	`0x30`	Actual RX frame length.

USB host and HPS block bridge

The USB host block exposes registers for DMA address/length, command, port control, frame, actual length, token, PHY, packet, retry and engine status. The HPS block bridge is the current practical way to pass the two USB images into the MiSTer/Verilator environment as block devices.