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Goes branchless on shadowing.

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This commit is contained in:
Thomas Harte 2020-10-27 21:56:03 -04:00
parent 20631a157b
commit 0c700094ea
1 changed files with 19 additions and 5 deletions
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24
Machines/Apple/AppleIIgs/MemoryMap.hpp
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@ -28,7 +28,7 @@ class MemoryMap {
void set_storage(std::vector<uint8_t> &ram, std::vector<uint8_t> &rom) {
// Keep a pointer for later; also note the proper RAM offset.
ram_base = ram.data();
shadow_offset = ram.size() - (128*1024);
shadow_base[1] = &ram[ram.size() - 128*1024];
// Establish bank mapping.
uint8_t next_region = 0;
@ -397,6 +397,8 @@ class MemoryMap {
set_card_paging();
}
uint8_t shadow_throwaway_;
public:
// Memory layout here is done via double indirection; the main loop should:
// (i) use the top two bytes of the address to get an index from memory_map_; and
@ -406,7 +408,8 @@ class MemoryMap {
// reduces what would otherwise be a 1.25mb table down to not a great deal more than 64kb.
std::array<uint8_t, 65536> region_map;
uint8_t *ram_base = nullptr;
size_t shadow_offset;
uint8_t *shadow_base[2] = {&shadow_throwaway_, nullptr};
const int shadow_modulo[2] = {1, 128*1024};
struct Region {
uint8_t *write = nullptr;
@ -430,11 +433,22 @@ class MemoryMap {
#define MemoryMapWrite(map, region, address, value) \
if(region.write) { \
region.write[address] = *value; \
if(region.flags & MemoryMap::Region::IsShadowed) { \
map.ram_base[(&region.write[address] - map.ram_base) % (128 * 1024) + map.shadow_offset] = *value; \
} \
const auto is_shadowed = region.flags & MemoryMap::Region::IsShadowed; \
map.shadow_base[is_shadowed][(&region.write[address] - map.ram_base) % map.shadow_modulo[is_shadowed]] = *value; \
}
// Quick notes on ::IsShadowed contortions:
//
// The objective is to support shadowing:
// 1. without storing a whole extra pointer, and such that the shadowing flags are orthogonal to the current auxiliary memory settings;
// 2. in such a way as to support shadowing both in banks $00/$01 and elsewhere; and
// 3. to do so without introducing too much in the way of branching.
//
// Hence the implemented solution: if shadowing is enabled then use the distance from the start of physical RAM
// modulo 128k indexed into the bank $e0/$e1 RAM.
//
// With a further twist: the modulo and pointer are indexed on ::IsShadowed to eliminate a branch even on that.
}
}