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Take another run at MEMC.

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I hadn't spotted that it is valid to map different logical pages to the same physical page with different protection levels.
This commit is contained in:
Thomas Harte 2024-03-27 10:44:40 -04:00
parent 166793ebe6
commit f38bca37a2
1 changed files with 52 additions and 35 deletions
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87
Machines/Acorn/Archimedes/MemoryController.hpp
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@ -294,12 +294,20 @@ struct MemoryController {
// Physical to logical mapping.
std::array<uint32_t, 128> pages_{};
// Logical to physical mapping.
struct MappedPage {
uint8_t *target = nullptr;
uint8_t protection_level = 0;
};
std::array<MappedPage, 8192> mapping_;
// Logical to physical mapping; this is divided by 'access mode'
// (i.e. the combination of read/write, trans and OS mode flags,
// as multipliexed by the @c mapping() function) because mapping
// varies by mode — not just in terms of restricting access, but
// actually presenting different memory.
using MapTarget = std::array<uint8_t *, 8192>;
std::array<MapTarget, 6> mapping_;
template <bool is_read>
MapTarget &mapping(bool trans, bool os_mode) {
const size_t index = (is_read ? 1 : 0) | (os_mode ? 2 : 0) | ((trans && !os_mode) ? 4 : 0);
return mapping_[index];
}
bool map_dirty_ = true;
template <typename IntT, bool is_read>
@ -310,7 +318,6 @@ struct MemoryController {
update_mapping();
map_dirty_ = false;
}
address = aligned<IntT>(address);
address &= 0x1ff'ffff;
size_t page;
@ -336,33 +343,11 @@ struct MemoryController {
break;
}
if(!mapping_[page].target) {
const auto &map = mapping<is_read>(trans, os_mode_);
if(!map[page]) {
return nullptr;
}
// TODO: eliminate switch here.
// Top of my head idea: is_read, trans and os_mode_ make three bits, so
// keep a one-byte bitmap of permitted accesses rather than the raw protection
// level?
if(trans) {
switch(mapping_[page].protection_level) {
case 0b00: break;
case 0b01:
// Reject: writes, in user mode.
if(!is_read && !os_mode_) {
return nullptr;
}
break;
default:
// Reject: writes, and user mode.
if(!is_read || !os_mode_) {
return nullptr;
}
break;
}
}
return reinterpret_cast<IntT *>(mapping_[page].target + address);
return reinterpret_cast<IntT *>(&map[page][address]);
}
void update_mapping() {
@ -379,7 +364,9 @@ struct MemoryController {
template <PageSize size>
void update_mapping() {
// Clear all logical mappings.
std::fill(mapping_.begin(), mapping_.end(), MappedPage{});
for(auto &map: mapping_) {
std::fill(map.begin(), map.end(), nullptr);
}
// For each physical page, project it into logical space
// and store it.
@ -445,8 +432,38 @@ struct MemoryController {
// TODO: consider clashes.
// TODO: what if there's less than 4mb present?
mapping_[logical].target = &ram_[physical];
mapping_[logical].protection_level = (page >> 8) & 3;
const auto target = &ram_[physical];
const auto set_supervisor = [&](bool read, bool write) {
if(read) mapping<true>(false, false)[logical] = target;
if(write) mapping<false>(false, false)[logical] = target;
};
const auto set_os = [&](bool read, bool write) {
if(read) mapping<true>(true, true)[logical] = target;
if(write) mapping<false>(true, true)[logical] = target;
};
const auto set_user = [&](bool read, bool write) {
if(read) mapping<true>(true, false)[logical] = target;
if(write) mapping<false>(true, false)[logical] = target;
};
set_supervisor(true, true);
switch((page >> 8) & 3) {
case 0b00:
set_os(true, true);
set_user(true, true);
break;
case 0b01:
set_os(true, true);
set_user(true, false);
break;
default:
set_os(true, false);
set_user(false, false);
break;
}
}
}
};