Memory The programmer’s model .text, .data .ktext, .kdata user program (instruction memory) and data area .ktext, .kdata kernel program and data area The computer’s model instruction memory user space 0-7fffffff, kernel space 80000000 - ffffffff data memory

How can we tell User/Kernel apart? 31 30 29 28 27 ? x x x x…... By looking at the address! PC[31] 0, User program 1, Kernel program data addr[31] 0, User data 1, Kernel data

Address Decoding So far; Instruction decoding Now address decoding 32 PC PC[31] TAP BOX [31]

How to detect a kernel instruction CPU model How to detect a kernel instruction kinst CP0 MIPS PIPELINE 32 instruction address [31] Instruction Memory

How to detect kernel data access CPU model How to detect kernel data access kdata CP0 MIPS PIPELINE data mem address 32 [31] Instruction Memory

Address Mapping CP0 MIPS PIPELINE data mem address 32 32 data 8/16/32 bits Address bus Data bus

Address Mapping CP0 MIPS PIPELINE Control signals r/w, 8, 16, 32 bits Address bus Data bus Control bus

Address Mapping Address bus Data bus Control bus 00……0 .. . 0fff…..f 10……0 .. . 1fff…..f n0……0 .. . nfff…..f …. Mem 1 Mem 2 Mem n

Enable and REDUCED address Address 0-7fffffff 31 30 29 28 27 [27:0] points out the REDUCED address in mem 1 mem 1 Enable

lw $t0 0($t1), $t1=0…0 Address bus Data bus Control bus read=(ADDR[31..28]=0000) and CTRL[READ] word=CTRL[WORD] 32 address decoder read, word 28 data word 00……0 .. . 0fff…..f

lw $t0 0($t1), $t1=0…0 Address bus Data bus Control bus CTRL[stall]=0 Until data ready 32 address decoder read, word 28 data word 00……0 .. . 0fff…..f

sb $t0 0($t1), $t1=0…0 Address bus Data bus Control bus write=(ADDR[31..28]=0000) and CTRL[WRITE] byte=CTRL[BYTE] 32 address decoder write, byte 28 data word 00……0 .. . 0fff…..f

sw $t0 0($t1), $t1=0…1 Address bus Data bus Control bus CTRL[ALIGNMENT]=(ADDR[31..28]=0000) and (ADDR[0] or ADDR[1]) 32 address decoder Alignment Error!!!