今日的行程：
辦公室：8:50 - 18:00
讀書室：19:00 - 20:30

正片開始

今日的記錄：

Condition flags

N: negative
Z: zero
C: carry
V: overflow

Add & Sub

Add 與其相似指令

// Add immediate
add x0, x1, #1            // x0 = x1 + 1
add x0, x1, #1, LSL #12    // x0 = x1 + (1 << 12)

// Add two registers
add x0, x1, x2            // x0 = x1 + x2
add x0, x1, x2, UXTB      // x0 = x1 + Unsigned_extend_byte(x2)

// Shifting add
add x0, x1, x2, LSL #3     // x0 = x1 + (x2 << 3)

// similiar: adds、adc
mov x1, 0xffff_ffff_ffff_ffff
adds x0, x1, #2           // x0 = x1 + 2, carry
mrs x2, nzcv              // x2 == 0x2000_0000 (29th bit: C)

mov x1, 0xffff_ffff_ffff_ffff
mov #2
adc x0, x1, x2            // x0 = x1 + 2 + C
mrs x3, nzcv              // C bit is set

Sub 與其相似指令 (和 Add 差不多)

一樣要注意的是 immediate 或是 shift 的大小

sub x0, x1, x2            // x0 = x1 - x2
sub x0, x1, x2, LSL #2     // x0 = x1 - (x2 << 2)

sub x0, x1, x2, SXTB      // x0 = x1 - Signed_extend_byte(x2)

sub x0, x1, x2, LSL #3     // x0 = x1 - (x2 << 3)

// subs
mov x1, #0x3
mov x2, #0x1
subs x0, x1, x2           // x0 = x1 + not(x2) + 1 = 0x3 + not(0x1) + 0x1
                          //    = 0x3 + 0xffff_ffff_ffff_fffe + 0x1 (carry) 
mrs x3, nzcv              // x3 == 0x2000_0000


mov x1, #0x3
mov x2, #0x3
subs x0, x1, x2           // x0 = 0x3 + not(0x3) + 0x1
                          //    = 0x3 + 0xffff_ffff_ffff_fffc + 0x1 = 0x0 (carry)
mrs x3, nzcv              // c,z bit both set

// sbc
mov x1, #3
mov x2, #1
sbc x0, x1, x2            // x0 = x1 + not(x2) + c = 0x3 + not(0x1) + c
                          //    = 0x3 + 0xffff_ffff_ffff_fffe + c (0x3 + 0xff... carry!, so c == 1)
                                = 0x3 + 0xffff_ffff_ffff_fffe + 1 = 0x2
mrs x3, nzcv              // c bit set

Shift operation

LSL: logic shift left
LSR: logic shift right (0 extend)
ASR: arithmetic shift right (sign extend)
ROR: rotate right

ldr w1, =0x8000_008a
asr w2, w1, 1            // w2 == 0xc000_0045
lsr w3, w1, 1            // w3 == 0x4000_0045

bitwise, bitfield operation

// and, ands
mov x1, #0x3
mov x2, #0
ands x3, x1, x2          // x3 = x1 & x2 = 0
mrs x0, nzcv             // z bit set

// orr, eor(exclusive or)

// bic (bit clear)
// clz (count leading zero)
// bfi, ubfx, sbfx
mov x0, #0
mov x1, #0x5
bfi x0, x1, #4, #4       // x0 的 bit [4+4-1, 4] = 0x5
            ^^^  ^^^
            lsb, width
            
mov x2, #0x8a
ubfx x0, x2, #4, #4      // x0 的 bit [7,4] = (0x8a) --> x0 == 0x8
sbfx x1, x2, #4, #4      // x1 的 bit [7,4] = 0x8a, 高位做 sign extend; 
                            x1 == 0xffff_ffff_ffff_fff8

總結

• 算術類指令，一樣是直上範例