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mem: Add comments for the DRAM address decoding

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This patch adds more verbose comments to explain the two different
address mapping schemes of the DRAM controller.
This commit is contained in:
Andreas Hansson 2013-01-31 07:49:18 -05:00
parent fce3433b2e
commit a4288dabf9
1 changed files with 25 additions and 0 deletions
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25
src/mem/simple_dram.cc
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@ -166,6 +166,11 @@ SimpleDRAM::writeQueueFull() const
SimpleDRAM::DRAMPacket*
SimpleDRAM::decodeAddr(PacketPtr pkt)
{
// decode the address based on the address mapping scheme
//
// with R, C, B and K denoting rank, column, bank and rank,
// respectively, and going from MSB to LSB, the two schemes are
// RKBC (openmap) and RCKB (closedmap)
uint8_t rank;
uint16_t bank;
uint16_t row;
@ -176,26 +181,46 @@ SimpleDRAM::decodeAddr(PacketPtr pkt)
// truncate the address to the access granularity
addr = addr / bytesPerCacheLine;
// we have removed the lowest order address bits that denote the
// position within the cache line, proceed and select the
// appropriate bits for bank, rank and row (no column address is
// needed)
if (addrMapping == Enums::openmap) {
// the lowest order bits denote the column to ensure that
// sequential cache lines occupy the same row
addr = addr / linesPerRowBuffer;
// after the column bits, we get the bank bits to interleave
// over the banks
bank = addr % banksPerRank;
addr = addr / banksPerRank;
// after the bank, we get the rank bits which thus interleaves
// over the ranks
rank = addr % ranksPerChannel;
addr = addr / ranksPerChannel;
// lastly, get the row bits
row = addr % rowsPerBank;
addr = addr / rowsPerBank;
} else if (addrMapping == Enums::closemap) {
// optimise for closed page mode and utilise maximum
// parallelism of the DRAM (at the cost of power)
// start with the bank bits, as this provides the maximum
// opportunity for parallelism between requests
bank = addr % banksPerRank;
addr = addr / banksPerRank;
// next get the rank bits
rank = addr % ranksPerChannel;
addr = addr / ranksPerChannel;
// next the column bits which we do not need to keep track of
// and simply skip past
addr = addr / linesPerRowBuffer;
// lastly, get the row bits
row = addr % rowsPerBank;
addr = addr / rowsPerBank;
} else