User:Gravaera/RISC-V Overview

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Riscv notes:

CSR=Control and Status Reg WIRI: Reserved Write Ignored, Reads Ignore Values: Hardware ignores writes to this field if RW; if RO, illegal exception generated. WPRI: Reserved Writes Preserve Values, Reads Ignore Values Hardware will commit write changes to this field, but software should not change the value in this field in the first place: preserve what you read out. WLRL: Write/Read Only Legal Values: Be careful to only write legal, and shouldn't assume that reads after illegal write will return legal values. WARL: Write Any, Read Legal Values: You can write illegal values and when read back, only legal values will be returned.

The Hart enters MMode on #RESET. Privilege level: 0=User, 1=Supervisor, 2=RSVD, 3=Machine; Machine mode must always exist. Implementations may also implement a "Debug" and "OEM Test" mode. No privlevel value is assigned by the spec. Since increasing privilege is encoded as increasingly higher numerical values, this is seamlessly doable. Machine mode should be trapped if it tries to access the D-CSRs [sec 2.1].

All privileged intrs are prefixed by the SYSTEM major opcode. Privileged instrs are 2 categories: (1) RMW ops to CSR regs, (2) All other privileged instrs.

CSRs: CSRs at level X are accessible to all levels higher than it (greater than X) as well. Shadow registers are RW versions of a lower priv reg which can be written to by a higher priv lvl. They enable higher priv determine what environment lower priv sees. Multiple higher privlvls may use the same shadow reg so shadow regs don't need to be provided to every privlvl for any given CSR.

* Attempting to access a non-existent CSR generates an illegal instr exception. * Accessing a CSR from a lower privlevel generates an illegal instr exception.

* There are 4096 CSRs altogether (12 bit encoding space). * 12 bit encoding space broken down into: CSR_IDX[11:10]: RW perms. (0x3=RO, 0x0-0x2=RW). CSR_IDX[9:8]: Lowest priv level which can access the CSR. Because increasing privilege is encoded using increasing value integers, this means that any privlevel CSR_IDX[9:8] or higher can access the CSR.

Multiple wordsizes: 32, 64, 128:

  • When CPU supports multiple wordsizes and is operating in wordsize lower than greatest
 mode supported, all operations must ignore source reg high bits sign extend result
 regs up to highest wordsize high bits.
  • Higher priv modes can query the current wordsize(XLEN) of a lower mode by reading
 xSTATUS.[yXLEN], where X is current privlvl and Y is privlvl to be queried about.

* See MISA CSR for how to detect MMode wordsize. * See MSTATUS CSR for how to control xXLEN of lower priv modes. ^ This means that MMode can support mixed wordsize guests (32, 64, 128). * See SSTATUS CSR for how to control UXLEN. ^ This means that SMode can support mixed wordsize binaries (32, 64, 128). * In all cases, the OEM is free to HARDWIRE values for xSTATUS.[xXL].

Machine Mode:

MISA CSR: Always readable without fault but returns 0 if not implemented. XLEN:XLEN-1: MXL | XLEN-3:26: RSVD. | 25:0: Extensions bits. MXL: Sets CPU ISA/reg wordsize: 0=unimplemented; 1=32; 2=64; 3=128 RW if multiple XLENs supported. The RISC-V spec gives advice on how to detect your current MXL using this reg without knowing beforehand. Extensions: WARL: Each bit from 0-25 represents a letter from A-Z. G-bit: Accesses separate namespace of new letters gA-gZ. G also means that ADFIM are implicitly supported. See riscv-misa-extension-bits.png. I: must be set; represents the base ISA. X: Non-standard extensions present. N: User-level traps. MVENDOR CSR: Always readable w/o fault but returns 0 if unimplemented or "non-commercial" vendor. MARCHID CSR: Always readable w/o fault but returns 0 if unimplemented. Bit XLEN (MSb): 0=open source vendor; 1=proprietary vendor. MIMPID CSR: Always readable w/o fault but returns 0 if unimplemented. MHARTID CSR: Always readable w/o fault. * Gives the HART ID. Not the Core ID. Hart IDs need not be contiguous. * At least one hard must have an ID of 0. Explicitly stated that this is BSP. MSTATUS CSR: Virtualization, xIntEn, xPrevIntEn, xPreviousPriv, xXLEN

  • Restricted copies of this reg appear in S and U Mode.
  • Bits are intended to be toggled with atomic instructions.

* See Trap Architecture section for how traps and xRET modify the xIE, xPIE and xPP bits. * Setting xXLEN sets the XLEN (wordsize) of the lower privlvl in question. ^ For systems whose highest wordsize is 32, MSTATUS.[xXL] does not exist. ^ In all cases, the OEM may HARDWIRE xSTATUS.[xXL] to some value. Bits: * Bit XLEN-1: SD bit. See FPU and other feature save and restore section. * Bit 16:13: FS and XS bits: Also see FPU and other feature save/restore section. * Bit 17: MPRV: See "Cute features" * Bit 19: MXR: See "Cute features" * Bit 20, 21, 22: See "Virtualization" section. MTVEC CSR: Must always be implemented but may contain hardwired RO value. * Is WARL so you can test to see if it is hardwired.

Vector base alignment: * Vector addr can be anywhere as long as it is 4B aligned for direct mode. * For Vectored mode, aligmment is 4*XLEN.

Bits: * 1:0: MODE: 0=Direct: all exceptions set PC to this.[BASE] 1=Vectored: Async exceptions set PC to (this.BASE + 4*CAUSE) MIDELEG/MEDELEG CSR: By default *ALL* traps route to MMode, but can be delegated to SMode.

  • If only MMode exists, or if only M+UMode exist without UMode-traps (N), then M?DELEG should NOT
 EXIST. I.e, accessing will trigger a fault.
  • Delegation from MMode is only to SMode. SMode can then choose to delegate to UMode if UMode traps
 are supported (MISA.Extensions[N]).
 ^ That said, if SMode doesn't exist (only M+U), then delegation goes to UMode.

MEDELEG: * Bit positions in MEDELEG correspond to exception cause values as used by MCAUSE. * MEDELEG[11] and SEDELEG[11:9] are hardwired to 0; CANNOT be delegated. MIDELEG: * Bit positions in MIDELEG correspond those of the MIP reg. MIP CSR: Machine Interrupt Pending ^ Has bits: MTIP/STIP/UTIP; MSIP/SSIP/USIP; MEIP/SEIP/UEIP.

  • See Interrupt architecture section for explanation of how these bits work.

MIE CSR: Machine Interrupt Enable:

  • See interrupt architecture section for explanation of how these bits work.

Supervisor Mode: Detected by MISA.Extensions[S].

UserMode: Detected by MISA.Extensions[U].

FLOATING POINT UNIT AND OTHER FEATURE SAVE/RESTORE: * Check FPU usage status when save/restoring using xSTATUS.[FS]. * Check extension feature usage status when save/restoring using xSTATUS.[XS].

When a feature is disabled, any attempt to access its regs/state will trigger an unspecified exception.

FS and XS field format: Both are 2 bits. 0=off: unit/features is off; 1=initial: feature is on and initialized, state unmodified since initialization; 2=clean: none dirty some state clean; 3=dirty: FPU or some features dirty. SD bit: This is a superbit which encompasses the state of both FS and XS.

CUTE FEATURES: Modified PRiVilege: Enable using xSTATUS.[MPRV]: Causes CPU to perform LOADS and STORES in mode X as if they were performed from mode xSTATUS.[xPP]. Basically set xPP and then you get to simulate data accesses (NOT INSTR FETCH+DECODE) as if executing in mode xPP.

Make eXecutable Readable: Enable using xSTATUS.[MXR]: RISCV CPUs will fault on data load from vmem pages without READ perms set by default. This feature makes it easier for a trap and emulate handler to emulate by allowing pages marked as EXEC only to also count as if they had READ set.

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