24.22.4.8.4. gem5 event queue DerivO3CPU syscall emulation freestanding example analysis: stall

We can see here that:

  • the addition of a data section entry changed our previous address setup a bit, the entry point was now 0x004000b0 which fits 4 instructions in the cacheline instead of 2

  • the LDR happens to be the fourth instruction, so it takes a long time to retire. The time is about 40k ticks, which is about the same time it takes for the instruction fetch as expected.

  • fetch does not continue past the LDR, and so nothing is gained in this particular example, since the next instructions haven’t been fetched from memory yet!

// f = fetch, d = decode, n = rename, p = dispatch, i = issue, c = complete, r = retire

                                     timeline                                             tick          pc.upc     disasm                      seq_num
[.ic.r........................................................................fdn]-(          40000) 0x004000b0.0 movz x0, #0, #0           [         1]
[.ic.r........................................................................fdn]-(          40000) 0x004000b4.0 movz x1, #1, #0           [         2]
[.ic.r........................................................................fdn]-(          40000) 0x004000b8.0 adr x2, #65780            [         3]
[.............................................................................fdn]-(          40000) 0x004000bc.0 ldr x3, [x2]              [         4]
[.pic............................................................................]-(          80000)     ...
[................................r...............................................]-(         120000)     ...
[....................fdn.ic......r...............................................]-(         120000) 0x004000c0.0 movz x4, #4, #0           [         5]
[....................fdn.ic......r...............................................]-(         120000) 0x004000c4.0 movz x5, #5, #0           [         6]
[....................fdn.ic......r...............................................]-(         120000) 0x004000c8.0 movz x6, #6, #0           [         7]
[....................fdn.ic......r...............................................]-(         120000) 0x004000cc.0 movz x7, #7, #0           [         8]
[....................fdn.ic......r...............................................]-(         120000) 0x004000d0.0 movz x8, #8, #0           [         9]
[....................fdn.ic......r...............................................]-(         120000) 0x004000d4.0 movz x9, #9, #0           [        10]
[....................fdn.pic.....r...............................................]-(         120000) 0x004000d8.0 movz x10, #10, #0         [        11]
[....................fdn.pic......r..............................................]-(         120000) 0x004000dc.0 movz x11, #11, #0         [        12]
[.....................fdn.ic......r..............................................]-(         120000) 0x004000e0.0 movz x12, #12, #0         [        13]
[.....................fdn.ic......r..............................................]-(         120000) 0x004000e4.0 movz x13, #13, #0         [        14]
[.....................fdn.ic......r..............................................]-(         120000) 0x004000e8.0 movz x14, #14, #0         [        15]
[.....................fdn.ic......r..............................................]-(         120000) 0x004000ec.0 movz x15, #15, #0         [        16]
[.....................fdn.pic.....r..............................................]-(         120000) 0x004000f0.0 movz x16, #16, #0         [        17]
[.....................fdn.pic.....r..............................................]-(         120000) 0x004000f4.0 movz x17, #17, #0         [        18]
[.....................fdn.pic.....r..............................................]-(         120000) 0x004000f8.0 movz x18, #18, #0         [        19]
[.....................fdn.pic......r.............................................]-(         120000) 0x004000fc.0 movz x19, #19, #0         [        20]