RISCV的浮点计算单元FPU

1.机器特征寄存器CSR_MISA

通过该寄存器可以查询当前处理器是否支持单精度浮点计算，是否支持双精度浮点计算，只要支持其中之一，就表示当前处理器支持浮点计算。

     csrr t0, CSR_MISAandi t0, t0, （COMPAT_HWCAP_ISA_F | COMPAT_HWCAP_ISA_D）beqz t0, out    //直接跳出表示机器硬件不支持浮点计算单元/*此处代码，表示机器硬件支持浮点计算单元*/    out:

2.机器状态寄存器CSR_STATUS

如果机器硬件上支持浮点计算单元，通过该寄存器可以启用或者关闭浮点计算功能。其中，设置相应位，就是启动浮点计算功能；清零相应位表示关闭浮点计算功能

/* 启用浮点计算功能 */li t0, SR_FScsrs CSR_STATUS, t0/* 关闭浮点计算功能 */li t0, SR_FScsrc CSR_STATUS, t0

3. 浮点计算单元

浮点控制状态寄存器fcsr
32个浮点寄存器f0/f1/.../f31

4.上下文保存

保存

ENTRY(__fstate_save)li  a2,  TASK_THREAD_F0add a0, a0, a2                      //此时， a0是线程信息结构首地址li t1, SR_FScsrs CSR_STATUS, t1                 //确保浮点计算单元启用frcsr t0                            //将浮点控制状态寄存器复制到通用寄存器t0中fsd f0,  TASK_THREAD_F0_F0(a0)      //先在线程信息结构体中保存32个浮点寄存器  fsd f1,  TASK_THREAD_F1_F0(a0)fsd f2,  TASK_THREAD_F2_F0(a0)fsd f3,  TASK_THREAD_F3_F0(a0)fsd f4,  TASK_THREAD_F4_F0(a0)fsd f5,  TASK_THREAD_F5_F0(a0)fsd f6,  TASK_THREAD_F6_F0(a0)fsd f7,  TASK_THREAD_F7_F0(a0)fsd f8,  TASK_THREAD_F8_F0(a0)fsd f9,  TASK_THREAD_F9_F0(a0)fsd f10, TASK_THREAD_F10_F0(a0)fsd f11, TASK_THREAD_F11_F0(a0)fsd f12, TASK_THREAD_F12_F0(a0)fsd f13, TASK_THREAD_F13_F0(a0)fsd f14, TASK_THREAD_F14_F0(a0)fsd f15, TASK_THREAD_F15_F0(a0)fsd f16, TASK_THREAD_F16_F0(a0)fsd f17, TASK_THREAD_F17_F0(a0)fsd f18, TASK_THREAD_F18_F0(a0)fsd f19, TASK_THREAD_F19_F0(a0)fsd f20, TASK_THREAD_F20_F0(a0)fsd f21, TASK_THREAD_F21_F0(a0)fsd f22, TASK_THREAD_F22_F0(a0)fsd f23, TASK_THREAD_F23_F0(a0)fsd f24, TASK_THREAD_F24_F0(a0)fsd f25, TASK_THREAD_F25_F0(a0)fsd f26, TASK_THREAD_F26_F0(a0)fsd f27, TASK_THREAD_F27_F0(a0)fsd f28, TASK_THREAD_F28_F0(a0)fsd f29, TASK_THREAD_F29_F0(a0)fsd f30, TASK_THREAD_F30_F0(a0)fsd f31, TASK_THREAD_F31_F0(a0)sw t0, TASK_THREAD_FCSR_F0(a0)            //在线程信息结构体中，保存浮点控制状态寄存器的值csrc CSR_STATUS, t1                        //关闭浮点计算单元ret
ENDPROC(__fstate_save)

恢复

ENTRY(__fstate_restore)li  a2,  TASK_THREAD_F0add a0, a0, a2li t1, SR_FSlw t0, TASK_THREAD_FCSR_F0(a0)csrs CSR_STATUS, t1                    //启用浮点计算单元fld f0,  TASK_THREAD_F0_F0(a0)        //恢复32个浮点寄存器fld f1,  TASK_THREAD_F1_F0(a0)fld f2,  TASK_THREAD_F2_F0(a0)fld f3,  TASK_THREAD_F3_F0(a0)fld f4,  TASK_THREAD_F4_F0(a0)fld f5,  TASK_THREAD_F5_F0(a0)fld f6,  TASK_THREAD_F6_F0(a0)fld f7,  TASK_THREAD_F7_F0(a0)fld f8,  TASK_THREAD_F8_F0(a0)fld f9,  TASK_THREAD_F9_F0(a0)fld f10, TASK_THREAD_F10_F0(a0)fld f11, TASK_THREAD_F11_F0(a0)fld f12, TASK_THREAD_F12_F0(a0)fld f13, TASK_THREAD_F13_F0(a0)fld f14, TASK_THREAD_F14_F0(a0)fld f15, TASK_THREAD_F15_F0(a0)fld f16, TASK_THREAD_F16_F0(a0)fld f17, TASK_THREAD_F17_F0(a0)fld f18, TASK_THREAD_F18_F0(a0)fld f19, TASK_THREAD_F19_F0(a0)fld f20, TASK_THREAD_F20_F0(a0)fld f21, TASK_THREAD_F21_F0(a0)fld f22, TASK_THREAD_F22_F0(a0)fld f23, TASK_THREAD_F23_F0(a0)fld f24, TASK_THREAD_F24_F0(a0)fld f25, TASK_THREAD_F25_F0(a0)fld f26, TASK_THREAD_F26_F0(a0)fld f27, TASK_THREAD_F27_F0(a0)fld f28, TASK_THREAD_F28_F0(a0)fld f29, TASK_THREAD_F29_F0(a0)fld f30, TASK_THREAD_F30_F0(a0)fld f31, TASK_THREAD_F31_F0(a0)fscsr t0                                    //恢复浮点控制状态寄存器csrc CSR_STATUS, t1                         //关闭浮点计算功能ret
ENDPROC(__fstate_restore)