BeagleBone PRU Notes

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The goal of this page is to record information that is pertinent to getting started with the BeagleBone's AM335x Programmable Real-time Unit (PRU).

BeagleBone Programmable Real-time Unit

This is also called the PRU Subsystem (PRUSS) or PRU and Industrial Controller Subsystem (PRU-ICSS). It is optimized to perform embedded tasks that require real-time constraints.

Most important is the purssdrv library to expose functions to the PRU. You can load this library by typing modprobe uio_pruss.

PRU capabilities


  • The PRU has dual 32-bit RISC cores, shared data and instruction memories and an interrupt controller (INTC).
  • 8KB data memory and 8KB instruction memory
  • 12KB shared RAM
  • A small, deterministic instruction set

There is no pipelining done on the processor and there are 29 (r1-r30) registers to use.

Data Sheet is located at

Software examples

BeagleBoard/TI has provided example C programs that utilize the PRU on github.

These include: PRU_memAccess_DDR_PRUsharedRAM, PRU_memAccessPRUDataRam, PRU_PRUtoPRUInterrupt, and a PRU assembler.

Full assembly guide is located at

Instruction Set

Nearly all instructions (with exception of accessing memory external to PRU) are single-cycle execute (5 ns when running at 200 MHz)

Four instruction classes

  • Arithmetic
  • Logical
  • Flow Control
  • Register Load/Store

Instruction Syntax

  • Mnemonic, followed by comma separated parameter list
  • Parameters can be a register, label, immediate value, or constant table entry
  • Example
    • SUB r3, r3, 10
    • Subtracts immediate value 10 (decimal) from the value in r3 and then places the result in r3

Example Assembly

;INTC_SIR_IRQ/INTC_SIR_FIQ register address
INTC_SIR_IRQ_ADDR/INTC_SIR_FIQ_ADDR .word 0x48200040/0x48200044
; ACTIVEIRQ bit field mask to get only the bit field
; Save the critical context
STMFD SP!, {R0-R12, LR} ; Save working registers and the Link register
MRS R11, SPSR ; Save the SPSR into R11
; Get the number of the highest priority active IRQ/FIQ
LDR R10, [R10] ; Get the INTC_SIR_IRQ/INTC_SIR_FIQ register
AND R10, R10, #ACTIVEIRQ_MASK ; Apply the mask to get the active IRQ number
; Jump to relevant subroutine handler
LDR PC, [PC, R10, lsl #2] ; PC base address points this instruction + 8
NOP ; To index the table by the PC
; Table of handler start addresses
.word IRQ0handler ;For IRQ0 of BANK0
.word IRQ1handler
.word IRQ2handler


Here's some sources of information.