mu - Soul of a tiny new machine. More thorough tests → More comprehensible and rewrite-friendly software → More resilient society.

	Commit message (Expand)	Author	Age	Files	Lines
*	1276 - make C++ version the default	Kartik K. Agaram	2015-05-05	1	-0/+6

28746b36 ^

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== code # Problem: create a function which pushes n zeros on the stack. # This is not a regular function, so it won't be idiomatic. # Registers must be properly restored. # Registers can be spilled, but that modifies the stack and needs to be # cleaned up. # This file is kinda like a research notebook, to interactively arrive at the # solution. Nobody should have to do this without a computer. To run it: # $ ./translate_subx_debug 000init.linux stack_array.subx && bootstrap/bootstrap --debug --trace --dump run a.elf # There are multiple versions. You'll need to uncomment exactly one. # The final version has its own Entry, but the others share this one. #? Entry: #? # . prologue #? 89/<- %ebp 4/r32/esp #? # #? 68/push 0xfcfdfeff/imm32 #? b8/copy-to-eax 0x34353637/imm32 #? $dump-stack: #? (push-n-zero-bytes 0x20) #? $dump-stack2: #? 68/push 0x20202020/imm32 #? $dump-stack3: #? b8/copy-to-eax 1/imm32/exit #? cd/syscall 0x80/imm8 ## 0 #? push-n-zero-bytes: # n: int #? # . prologue #? 55/push-ebp #? 89/<- %ebp 4/r32/esp #? $push-n-zero-bytes:end: #? # . epilogue #? 89/<- %esp 5/r32/ebp #? 5d/pop-to-ebp #? c3/return # stack at dump-stack: # 0 a: bdffffd0: 00000000 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000000 fcfdfeff 00000001 bf000000 # # => # # stack at dump-stack3: # 0 a: stack: # 0 a: bdffffd0: 00000000 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 bdfffff8/ebp 090000cc/ra # 0 a: bdfffff0: 00000004/arg fcfdfeff 00000001 bf000000 ## 1 #? push-n-zero-bytes: # n: int #? # . prologue #? 55/push-ebp #? 89/<- %ebp 4/r32/esp #? # . save registers #? 50/push-eax #? $push-n-zero-bytes:end: #? # . restore registers #? 58/pop-to-eax #? # . epilogue #? 5d/pop-to-ebp #? c3/return # stack at dump-stack3: # 0 a: bdffffd0: 00000000 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 34353637 bdfffff8 090000d1 # 0 a: bdfffff0: 00000004 fcfdfeff 00000001 bf000000 ## 2 #? push-n-zero-bytes: # n: int #? # . prologue #? 55/push-ebp #? 89/<- %ebp 4/r32/esp #? # . save registers #? 50/push-eax #? # #? 8b/-> *(esp+8) 0/r32/eax #? 2b/subtract *(ebp+8) 4/r32/esp #? 89/<- *(esp+8) 0/r32/eax #? $push-n-zero-bytes:end: #? # . restore registers #? 58/pop-to-eax #? # . epilogue #? 5d/pop-to-ebp #? c3/return # stack at dump-stack3: # 0 a: bdffff90: 00000000 00000000 00000000 00000000 # 0 a: bdffffa0: 00000000 00000000 00000000 00000000 # 0 a: bdffffb0: 00000000 00000000 00000000 00000000 # 0 a: bdffffc0: 00000000 00000000 00000000 090000d1 # 0 a: bdffffd0: 00000000 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 34353637 bdfffff8 090000d1 # 0 a: bdfffff0: 00000020 fcfdfeff 00000001 bf000000 ## 3 #? push-n-zero-bytes: # n: int #? # . prologue #? 55/push-ebp #? 89/<- %ebp 4/r32/esp #? # . save registers #? # -- esp = ebp #? 50/push-eax #? # -- esp+8 = ebp+4 #? 8b/-> *(esp+8) 0/r32/eax #? 2b/subtract *(ebp+8) 4/r32/esp #? 89/<- *(esp+8) 0/r32/eax #? c7 0/subop/copy *(ebp+4) 0/imm32 #? $push-n-zero-bytes:end: #? # . restore registers #? 58/pop-to-eax #? # . epilogue #? 5d/pop-to-ebp #? c3/return # stack at dump-stack3: # 0 a: bdffff90: 00000000 00000000 00000000 00000000 # 0 a: bdffffa0: 00000000 00000000 00000000 00000000 # 0 a: bdffffb0: 00000000 00000000 00000000 00000000 # 0 a: bdffffc0: 00000000 00000000 00000000 090000d1 # 0 a: bdffffd0: 20202020 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 34353637 bdfffff8 00000000 # 0 a: bdfffff0: 00000020 fcfdfeff 00000001 bf000000 ## 4 #? push-n-zero-bytes: # n: int #? # . prologue #? 55/push-ebp #? 89/<- %ebp 4/r32/esp #? # . save registers #? # -- esp = ebp #? 50/push-eax #? # copy return address over #? # -- esp+8 = ebp+4 #? 8b/-> *(esp+8) 0/r32/eax #? 2b/subtract *(ebp+8) 4/r32/esp #? 89/<- *(esp+8) 0/r32/eax #? 58/pop-to-eax #? c7 0/subop/copy *(ebp+8) 0/imm32 #? c7 0/subop/copy *(ebp+4) 0/imm32 #? c7 0/subop/copy *(ebp+0) 0/imm32 #? c7 0/subop/copy *(ebp-4) 0/imm32 #? # . epilogue #? 5d/pop-to-ebp #? c3/return # stack at dump-stack3: # 0 a: bdffff90: 00000000 00000000 00000000 00000000 # 0 a: bdffffa0: 00000000 00000000 00000000 00000000 # 0 a: bdffffb0: 00000000 00000000 00000000 00000000 # 0 a: bdffffc0: 00000000 00000000 00000000 090000d1 # 0 a: bdffffd0: 20202020 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000000 fcfdfeff 00000001 bf000000 # Stack looks good now (the 20202020 marks where the array length 0x20 will # go, and the next 0x20 bytes show the space for the array has been zeroed # out). # Final issue: ebp has been clobbered on return. ## 5 # I'd like to translate ebp to esp so we can stop pushing ebp. But we need to # hold 'n' somewhere, which would require a register, which we then need to # push. #? push-n-zero-bytes: # n: int #? 55/push-ebp #? 89/<- %ebp 4/r32/esp #? # -- esp = ebp #? 50/push-eax #? $push-n-zero-bytes:bulk-cleaning: #? $push-n-zero-bytes:copy-ra: #? # -- esp+8 = ebp+4 #? 8b/-> *(esp+8) 0/r32/eax #? 2b/subtract *(esp+0xc) 4/r32/esp #? # -- esp+8+n = ebp+4 #? 89/<- *(esp+8) 0/r32/eax #? 58/pop-to-eax #? # -- esp+n = ebp #? $push-n-zero-bytes:spot-cleaning: #? c7 0/subop/copy *(ebp+8) 0/imm32 #? c7 0/subop/copy *(ebp+4) 0/imm32 #? c7 0/subop/copy *(ebp+0) 0/imm32 #? c7 0/subop/copy *(ebp-4) 0/imm32 #? 5d/pop-to-ebp #? c3/return # stack at dump-stack3: # 0 a: bdffff90: 00000000 00000000 00000000 00000000 # 0 a: bdffffa0: 00000000 00000000 00000000 00000000 # 0 a: bdffffb0: 00000000 00000000 00000000 00000000 # 0 a: bdffffc0: 00000000 00000000 00000000 090000d1 # 0 a: bdffffd0: 20202020 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000000 fcfdfeff 00000001 bf000000 # Bah. May be simpler to just create a new segment of global space for this # function. ## 6 #? push-n-zero-bytes: # n: int #? 89/<- *Push-n-zero-bytes-ebp 5/r32/ebp # spill ebp without affecting stack #? 89/<- %ebp 4/r32/esp #? # -- esp = ebp #? 50/push-eax #? $push-n-zero-bytes:bulk-cleaning: #? $push-n-zero-bytes:copy-ra: #? # -- esp+8 = ebp+4 #? # -- esp+4 = ebp #? 8b/-> *(esp+4) 0/r32/eax #? 2b/subtract *(ebp+4) 4/r32/esp #? # -- esp+4+n = ebp #? 89/<- *(esp+4) 0/r32/eax #? 58/pop-to-eax #? # -- esp+n = ebp #? $push-n-zero-bytes:spot-cleaning: #? c7 0/subop/copy *(ebp+4) 0/imm32 #? c7 0/subop/copy *(ebp+0) 0/imm32 #? c7 0/subop/copy *(ebp-4) 0/imm32 #? c7 0/subop/copy *(ebp-8) 0/imm32 #? 8b/-> *Push-n-zero-bytes-ebp 5/r32/ebp # restore spill #? c3/return #? #? == data #? Push-n-zero-bytes-ebp: # (addr int) #? 0/imm32 #? == code # stack at dump-stack3: # 0 a: bdffff90: 00000000 00000000 00000000 00000000 # 0 a: bdffffa0: 00000000 00000000 00000000 00000000 # 0 a: bdffffb0: 00000000 00000000 00000000 00000000 # 0 a: bdffffc0: 00000000 00000000 00000000 090000d1 # 0 a: bdffffd0: 20202020 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000000 fcfdfeff 00000001 bf000000 # Ok, we're there. Now start using zero-out rather than spot-cleaning. ## 7: we need to zero out the return address, but we can't do it inside the function. ## So we'll change the signature slightly. ## Before: clear N bytes and then push N as the array length. ## After: clear N bytes, set *esp to N. ## The helper adds and clears N bytes *before* esp. esp can't be cleared since ## it contains the return address. #? Entry: #? # . prologue #? 89/<- %ebp 4/r32/esp #? # #? 68/push 0xfcfdfeff/imm32 #? b8/copy-to-eax 0x34353637/imm32 #? $dump-stack0: #? (push-n-zero-bytes 0x20) #? $dump-stack9: #? c7 0/subop/copy *esp 0x20/imm32 #? $dump-stacka: #? b8/copy-to-eax 1/imm32/exit #? cd/syscall 0x80/imm8 #? #? push-n-zero-bytes: # n: int #? $push-n-zero-bytes:prologue: #? 89/<- *Push-n-zero-bytes-ebp 5/r32/ebp # spill ebp without affecting stack #? 89/<- %ebp 4/r32/esp #? $push-n-zero-bytes:copy-ra: #? $dump-stack1: #? # -- esp = ebp #? 50/push-eax #? $dump-stack2: #? # -- esp+8 = ebp+4 #? # -- esp+4 = ebp #? 8b/-> *(esp+4) 0/r32/eax #? $dump-stack3: #? 2b/subtract *(ebp+4) 4/r32/esp #? $dump-stack4: #? # -- esp+4+n = ebp #? 89/<- *(esp+4) 0/r32/eax #? $dump-stack5: #? 58/pop-to-eax #? # -- esp+n = ebp #? $push-n-zero-bytes:bulk-cleaning: #? $dump-stack6: #? 89/<- *Push-n-zero-bytes-esp 4/r32/esp #? 81 0/subop/add *Push-n-zero-bytes-esp 4/imm32 #? $dump-stack7: #? (zero-out *Push-n-zero-bytes-esp *(ebp+4)) # n #? $push-n-zero-bytes:epilogue: #? $dump-stack8: #? 8b/-> *Push-n-zero-bytes-ebp 5/r32/ebp # restore spill #? c3/return #? #? zero-out: # start: (addr byte), len: int #? # pseudocode: #? # curr/esi = start #? # i/ecx = 0 #? # while true #? # if (i >= len) break #? # *curr = 0 #? # ++curr #? # ++i #? # #? # . prologue #? 55/push-ebp #? 89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp #? # . save registers #? 50/push-eax #? 51/push-ecx #? 52/push-edx #? 56/push-esi #? # curr/esi = start #? 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 8/disp8 . # copy *(ebp+8) to esi #? # var i/ecx: int = 0 #? 31/xor 3/mod/direct 1/rm32/ecx . . . 1/r32/ecx . . # clear ecx #? # edx = len #? 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 2/r32/edx 0xc/disp8 . # copy *(ebp+12) to edx #? $zero-out:loop: #? # if (i >= len) break #? 39/compare 3/mod/direct 1/rm32/ecx . . . 2/r32/edx . . # compare ecx with edx #? 7d/jump-if->= $zero-out:end/disp8 #? # *curr = 0 #? c6 0/subop/copy 0/mod/direct 6/rm32/esi . . . . . 0/imm8 # copy byte to *esi #? # ++curr #? 46/increment-esi #? # ++i #? 41/increment-ecx #? eb/jump $zero-out:loop/disp8 #? $zero-out:end: #? # . restore registers #? 5e/pop-to-esi #? 5a/pop-to-edx #? 59/pop-to-ecx #? 58/pop-to-eax #? # . epilogue #? 89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp #? 5d/pop-to-ebp #? c3/return #? #? == data #? Push-n-zero-bytes-ebp: # (addr int) #? 0/imm32 #? Push-n-zero-bytes-esp: # (addr int) #? 0/imm32 #? == code # stack at dump-stack0: # 0 a: bdffffb0: 00000000 00000000 00000000 00000000 # 0 a: bdffffc0: 00000000 00000000 00000000 00000000 # 0 a: bdffffd0: 00000000 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000000 [fcfdfeff] 00000001 bf000000 # desired state after push-n-zero-bytes: # 0 a: bdffff90: 00000000 00000000 00000000 00000000 # 0 a: bdffffa0: 00000000 00000000 00000000 00000000 # 0 a: bdffffb0: 00000000 00000000 00000000 bdffffec # 0 a: bdffffc0: 0900012a bdffffd0 00000020 090000d1 # 0 a: bdffffd0: [rrrrrrrr] 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000000 fcfdfeff 00000001 bf000000 # Stack pointer contains ra is caller's responsibility to over-write with array length. # actual state: # 0 a: bdffff90: 00000000 00000000 00000000 00000000 # 0 a: bdffffa0: 00000000 00000000 00000000 00000000 # 0 a: bdffffb0: 00000000 00000000 00000000 bdffffec # 0 a: bdffffc0: 0900012a bdffffd0 00000020 090000d1 # 0 a: bdffffd0: 00000000 [00000000] 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000020 fcfdfeff 00000001 bf000000 # Couple of issues. But where does the return address disappear to? ## 8: #? Entry: #? # . prologue #? 89/<- %ebp 4/r32/esp #? # #? 68/push 0xfcfdfeff/imm32 #? b8/copy-to-eax 0x34353637/imm32 #? $dump-stack0: #? (push-n-zero-bytes 0x20) #? $dump-stack9: #? 68/push 0x20/imm32 #? #? c7 0/subop/copy *esp 0x20/imm32 #? $dump-stacka: #? b8/copy-to-eax 1/imm32/exit #? cd/syscall 0x80/imm8 #? #? push-n-zero-bytes: # n: int #? $push-n-zero-bytes:prologue: #? 89/<- *Push-n-zero-bytes-ebp 5/r32/ebp # spill ebp without affecting stack #? 89/<- %ebp 4/r32/esp #? $push-n-zero-bytes:copy-ra: #? $dump-stack1: #? # -- esp = ebp #? 50/push-eax #? $dump-stack2: #? # -- esp+8 = ebp+4 #? # -- esp+4 = ebp #? 8b/-> *(esp+4) 0/r32/eax #? $dump-stack3: #? 2b/subtract *(ebp+4) 4/r32/esp #? $dump-stack4: #? # -- esp+4+n = ebp #? 89/<- *(esp+4) 0/r32/eax #? $dump-stack5: #? 58/pop-to-eax #? # -- esp+n = ebp #? $push-n-zero-bytes:bulk-cleaning: #? $dump-stack6: #? 89/<- *Push-n-zero-bytes-esp 4/r32/esp #? 81 0/subop/add *Push-n-zero-bytes-esp 4/imm32 #? $dump-stack7: #? (zero-out *Push-n-zero-bytes-esp *(ebp+4)) # n #? $push-n-zero-bytes:epilogue: #? $dump-stack8: #? 8b/-> *Push-n-zero-bytes-ebp 5/r32/ebp # restore spill #? c3/return #? #? zero-out: # start: (addr byte), len: int #? # pseudocode: #? # curr/esi = start #? # i/ecx = 0 #? # while true #? # if (i >= len) break #? # *curr = 0 #? # ++curr #? # ++i #? # #? # . prologue #? 55/push-ebp #? 89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp #? # . save registers #? 50/push-eax #? 51/push-ecx #? 52/push-edx #? 56/push-esi #? # curr/esi = start #? 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 8/disp8 . # copy *(ebp+8) to esi #? # var i/ecx: int = 0 #? 31/xor 3/mod/direct 1/rm32/ecx . . . 1/r32/ecx . . # clear ecx #? # edx = len #? 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 2/r32/edx 0xc/disp8 . # copy *(ebp+12) to edx #? $zero-out:loop: #? # if (i >= len) break #? 39/compare 3/mod/direct 1/rm32/ecx . . . 2/r32/edx . . # compare ecx with edx #? 7d/jump-if->= $zero-out:end/disp8 #? # *curr = 0 #? c6 0/subop/copy 0/mod/direct 6/rm32/esi . . . . . 0/imm8 # copy byte to *esi #? # ++curr #? 46/increment-esi #? # ++i #? 41/increment-ecx #? eb/jump $zero-out:loop/disp8 #? $zero-out:end: #? # . restore registers #? 5e/pop-to-esi #? 5a/pop-to-edx #? 59/pop-to-ecx #? 58/pop-to-eax #? # . epilogue #? 89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp #? 5d/pop-to-ebp #? c3/return #? #? == data #? Push-n-zero-bytes-ebp: # (addr int) #? 0/imm32 #? Push-n-zero-bytes-esp: # (addr int) #? 0/imm32 #? == code # stack at dump-stack0: # 0 a: bdffffb0: 00000000 00000000 00000000 00000000 # 0 a: bdffffc0: 00000000 00000000 00000000 00000000 # 0 a: bdffffd0: 00000000 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000000 [fcfdfeff] 00000001 bf000000 # desired state after push-n-zero-bytes: # 0 a: bdffff90: 00000000 00000000 00000000 00000000 # 0 a: bdffffa0: 00000000 00000000 00000000 00000000 # 0 a: bdffffb0: 00000000 00000000 00000000 bdffffec # 0 a: bdffffc0: 0900012a bdffffd0 00000020 090000d1 # 0 a: bdffffd0: [rrrrrrrr] 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000000 fcfdfeff 00000001 bf000000 # actual state: # 0 a: bdffff90: 00000000 00000000 00000000 00000000 # 0 a: bdffffa0: 00000000 00000000 00000000 00000000 # 0 a: bdffffb0: 00000000 00000000 00000000 bdffffec # 0 a: bdffffc0: 09000124 bdffffd0 00000020 090000d1 # 0 a: bdffffd0: [00000000] 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000020 fcfdfeff 00000001 bf000000 # Ok, just one diff, at bdfffff0 ## 9: Entry: # . prologue 89/<- %ebp 4/r32/esp # 68/push 0xfcfdfeff/imm32 b8/copy-to-eax 0x34353637/imm32 $dump-stack0: (push-n-zero-bytes 0x20) $dump-stack9: 68/push 0x20/imm32 $dump-stacka: b8/copy-to-eax 1/imm32/exit cd/syscall 0x80/imm8 push-n-zero-bytes: # n: int $push-n-zero-bytes:prologue: 89/<- *Push-n-zero-bytes-ebp 5/r32/ebp # spill ebp without affecting stack 89/<- %ebp 4/r32/esp $push-n-zero-bytes:copy-ra: $dump-stack1: # -- esp = ebp 50/push-eax $dump-stack2: # -- esp+8 = ebp+4 # -- esp+4 = ebp 8b/-> *(esp+4) 0/r32/eax $dump-stack3: 2b/subtract *(ebp+4) 4/r32/esp $dump-stack4: # -- esp+4+n = ebp 89/<- *(esp+4) 0/r32/eax $dump-stack5: 58/pop-to-eax # -- esp+n = ebp $push-n-zero-bytes:bulk-cleaning: $dump-stack6: 89/<- *Push-n-zero-bytes-esp 4/r32/esp 81 0/subop/add *Push-n-zero-bytes-esp 4/imm32 $dump-stack7: 81 0/subop/add *(ebp+4) 4/imm32 (zero-out *Push-n-zero-bytes-esp *(ebp+4)) # n $push-n-zero-bytes:epilogue: $dump-stack8: 8b/-> *Push-n-zero-bytes-ebp 5/r32/ebp # restore spill c3/return zero-out: # start: (addr byte), len: int # pseudocode: # curr/esi = start # i/ecx = 0 # while true # if (i >= len) break # *curr = 0 # ++curr # ++i # # . prologue 55/push-ebp 89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp # . save registers 50/push-eax 51/push-ecx 52/push-edx 56/push-esi # curr/esi = start 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 8/disp8 . # copy *(ebp+8) to esi # var i/ecx: int = 0 31/xor 3/mod/direct 1/rm32/ecx . . . 1/r32/ecx . . # clear ecx # edx = len 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 2/r32/edx 0xc/disp8 . # copy *(ebp+12) to edx $zero-out:loop: # if (i >= len) break 39/compare 3/mod/direct 1/rm32/ecx . . . 2/r32/edx . . # compare ecx with edx 7d/jump-if->= $zero-out:end/disp8 # *curr = 0 c6 0/subop/copy 0/mod/direct 6/rm32/esi . . . . . 0/imm8 # copy byte to *esi # ++curr 46/increment-esi # ++i 41/increment-ecx eb/jump $zero-out:loop/disp8 $zero-out:end: # . restore registers 5e/pop-to-esi 5a/pop-to-edx 59/pop-to-ecx 58/pop-to-eax # . epilogue 89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp 5d/pop-to-ebp c3/return == data Push-n-zero-bytes-ebp: # (addr int) 0/imm32 Push-n-zero-bytes-esp: # (addr int) 0/imm32 == code # stack at dump-stack0: # 0 a: bdffffb0: 00000000 00000000 00000000 00000000 # 0 a: bdffffc0: 00000000 00000000 00000000 00000000 # 0 a: bdffffd0: 00000000 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000000 [fcfdfeff] 00000001 bf000000 # desired state after push-n-zero-bytes: # 0 a: bdffff90: 00000000 00000000 00000000 00000000 # 0 a: bdffffa0: 00000000 00000000 00000000 00000000 # 0 a: bdffffb0: 00000000 00000000 00000000 bdffffec # 0 a: bdffffc0: 0900012a bdffffd0 00000020 090000d1 # 0 a: bdffffd0: [xxxxxxxx] 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000000 fcfdfeff 00000001 bf000000 # actual state: # 0 a: bdffff90: 00000000 00000000 00000000 00000000 # 0 a: bdffffa0: 00000000 00000000 00000000 00000000 # 0 a: bdffffb0: 00000000 00000000 00000000 bdffffec # 0 a: bdffffc0: 0900012f bdffffd0 00000024 090000d1 # 0 a: bdffffd0: [00000000] 00000000 00000000 00000000 # 0 a: bdffffe0: 00000000 00000000 00000000 00000000 # 0 a: bdfffff0: 00000000 fcfdfeff 00000001 bf000000