Language of the Computer: Branch and Jump Addressing

Instructions: Language of the Computer

Constant

-          Kebanyakan constant adalah kecil

-          16-bit immediate adalah mencukupi

-          constant kecil kerap digunakan(50% daripada operan)

-          e.g. 

·         A = A + 5;

·         B = B + 1;

·         C = C - 18;

-          Jadi, dalam kebanyakan program, constant akan dimuatkan dalam 16 bit yang diperuntukkan bagi immediate field

-          Design Principle: Make the common case fast

– Common case: constant is small

–Hanya perlu menggunakan satu arahan dalam common case

Untuk constant yang besar

-          Kita dapat memuatkan 32-bit ke dalam register

-          Mesti guna dua instuction, new "load upper immediate" instruction

-          For the occasional 32-bit constant

            lui rt, constant

·         Copies 16-bit constant to left 16 bits of rt

·         Clears right 16 bits of rt to 0

Branch addressing

-          Arahan cabang adalah dinyatakan

·         Opcode, two registers, target address

-          Most branch targets are near branch

-          Formats:

-          dapat menentukan register (seperti LW dan Sw) dan menambah ia untuk menangani

·         penggunaan Instruction Address Register (PC = program counter)

·         Kebanyakan branch adalah local (principle of locality)

PC-relative addressing

-          Immediate field  adalah 16 bit tetapi kita perlu address yang 32 bit

-          Obtain address using PC-relative addressing

·         On branch,  new PC = PC + immediate field in branch instruction

·         Actually, new PC = (PC + 4) + immediate field in branch instruction

Jump addressing

-          Jump (j and jal) targets could be anywhere in text segment

·         Encode full address in instruction

-          J format ada 26 bits dalam address field

-          Untuk mendapatkan 32 bits

·         Anggapkan bahawa jump address adalah word address

·         26 + 2 (least significant bits) = 28

·         Get 4 most significant bits from PC

·         4 + 26 + 2 = 32

·         Hanya boleh jump antara   2^28= 256 MB block of addresses

-          (Pseudo)Direct jump addressing

·         Target address = PC_31…28 : (address × 4)

Target Addressing Example

Branching Far Away

Addressing Mode Summary