11. Low-Level Core Control

This chapter describes some low-level core control registers.

11.1. Control/Status Registers

A summary of platform-specific control/status registers in CSR space:

All reserved and unused bits in these control/status registers must be hardwired to ‘0’. Unless otherwise noted, all read/write control/status registers must have WARL (Write Any value, Read Legal value) behavior.

11.1.1. Feature Disable Control Register (mfdc)

The mfdc register hosts low-level core control bits to disable specific features. This may be useful in case a feature intended to increase core performance should prove to have problems.

Note

fence.i instructions are required before and after writes to the mfdc register.

Note

The default state of the controllable features is ‘enabled’. Firmware may turn off a feature if needed.

This register is mapped to the non-standard read/write CSR address space.

Table 11.1 Feature Disable Control Register (mfdc, at CSR 0x7F9). Field Bits Description

Field

Bits

Description

Access

Reset

Reserved

31:19

Reserved

R

0

dqc

18:16

DMA QoS control, see Quality Of Service

R/W

7

Reserved

15:13

Reserved

R

0

td

12

Trace disable:

  • 0: enable trace

  • 1: disable trace

R/W

0

elfd

11

External load-to-load forwarding disable:

  • 0: enable external load-to-load forwarding

  • 1: disable external load-to-load forwarding

R/W

0

Reserved

10:9

Reserved

R

0

cecd

8

Core ECC check disable:

  • 0: ICCM/DCCM ECC checking enabled

  • 1: ICCM/DCCM ECC checking disabled

R/W

0

Reserved

7

Reserved

R

0

sepd

6

Side effect pipelining disable:

  • 0: side effect loads/stores are pipelined

  • 1: side effect loads/stores block all subsequent bus transactions until load/store response with default value received.

    Note: Reset value depends on selected bus core build argument

R/W

0 (AHB-Lite) / 1 (AXI4)

Reserved

5:4

Reserved

R

0

bpd

3

Branch prediction disable:

  • 0: enable branch prediction and return address stack

  • 1: disable branch prediction and return address stack

R/W

0

wbcd

2

Write Buffer (WB) coalescing disable:

  • 0: enable Write Buffer coalescing

  • 1: disable Write Buffer coalescing

R/W

0

Reserved

1

Reserved

R

0

pd

0

Pipelining disable:

  • 0: pipelined execution

  • 1: single instruction execution

R/W

0

11.1.2. Clock Gating Control Register (mcgc)

The mcgc register hosts low-level core control bits to override clock gating for specific units. This may be useful in case a unit intended to be clock gated should prove to have problems when in lower power mode.

Note

Except for PIC I/O, the default state of the clock gating overrides is ‘disabled’. Firmware may turn off clock gating (i.e., set the clock gating override bit) for a specific unit if needed.

This register is mapped to the non-standard read/write CSR address space.

Table 11.2 Clock Gating Control Register (mcgc, at CSR 0x7F8). Field Bits Description

Field

Bits

Description

Access

Reset

Reserved

31:10

Reserved

R

0

picio

9

PIC I/O clock gating override:

  • 0: enable clock gating

  • 1: clock gating override

R/W

1

misc

8

Miscellaneous clock gating override:

  • 0: enable clock gating

  • 1: clock gating override

R/W

0

dec

7

DEC clock gating override:

  • 0: enable clock gating

  • 1: clock gating override

R/W

0

exu

6

EXU clock gating override:

  • 0: enable clock gating

  • 1: clock gating override

R/W

0

ifu

5

IFU clock gating override:

  • 0: enable clock gating

  • 1: clock gating override

R/W

0

lsu

4

LSU clock gating override:

  • 0: enable clock gating

  • 1: clock gating override

R/W

0

bus

3

Bus clock gating override:

  • 0: enable clock gating

  • 1: clock gating override

R/W

0

pic

2

PIC clock gating override:

  • 0: enable clock gating

  • 1: clock gating override

R/W

0

dccm

1

DCCM clock gating override:

  • 0: enable clock gating

  • 1: clock gating override

R/W

0

iccm

0

ICCM clock gating override:

  • 0: enable clock gating

  • 1: clock gating override

R/W

0
Last update: 2024-09-10