第2章 Risc-V ISA¶

operation¶

Risc-V的arithmetic指令的操作数必须是register

Risc-V Registers

Name	Register Name	Usage	Preserved or call?
x0 (zero)	0	The constant value 0	n.a.
x1 (ra)	1	Return address(link register)	yes
x2 (sp)	2	Stack pointer	yes
x3 (gp)	3	Global pointer	yes
x4 (tp)	4	Thread pointer	yes
x5-x7 (t0-t2)	5-7	Temporaries	no
x8 (s0/fp)	8	Saved/frame pointer	yes
x9 (s1)	9	Saved	yes
x10-x17 (a0-a7)	10-17	Arguments/results	no
x18-x27 (s2-s11)	18-27	Saved	yes
x28-x31 (t3-t6)	28-31	Temporaries	no
PC	-	Add Upper Immediate to PC	yes

根据教材的硬件设施，有32个64bit的register，memory中的数据以1个doubleword (8Byte = 64bit) 为一个访问单位，每一次访问memory都是访问一个doubleword块，因此访问起始地址为 0, 8, 16, 24...以此类推

主存(main memory)用于存放复杂数据结构如Arrays, structures, dynamic data
进行算术运算通过
- Load values from memory into registers
- Store result from register to memory
Memory is byte addressed Each address identifies an 8-bit byte
RISC-V is Little Endian
- 小端法：Least-significant byte at least address of a word
- 大端法: most-significant byte at least address
RISC-V does not require words to be aligned in memory words align: 一个字是 4 字节，我们要求字的起始地址一定要是 4 的倍数。

Registers are faster to access than memory
Operating on memory data requires loads and stores
Compiler must use registers for variables as much as possible 编译器尽量使用寄存器存变量。只有在寄存器不够用时，才会把不太用的值放回内存。

Risc-V ISA

memory operand example

Casm

A[12] = h + A[8];
/* assume A is an array of 8Byte elements (a doubleword),
Base of A is stored in x22, h in x21 */

ld x9, 64(x22)
add x9, x21, x9
sd x9, 96(x22)