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Switch to specifying number of bits, to reduce potential error.

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This commit is contained in:
Thomas Harte 2023-02-02 12:12:11 -05:00
parent debdad350d
commit 6d315b4660
2 changed files with 20 additions and 4 deletions
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5
Components/9918/Implementation/9918.cpp
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@ -929,7 +929,10 @@ void Base<personality>::write_register(uint8_t value) {
// The RAM pointer is always set on a second write, regardless of
// whether the caller is intending to enqueue a VDP operation.
install_field<8, 0x3f>(ram_pointer_, value);
//
// The top two bits are used to determine the type of write; only
// the lower six are actual address.
install_field<8, 6>(ram_pointer_, value);
write_phase_ = false;
if(value & 0x80) {

19
Components/9918/Implementation/9918Base.hpp
View File

@ -450,10 +450,12 @@ template <Personality personality> struct Base: public Storage<personality> {
TVStandard tv_standard_ = TVStandard::NTSC;
using AddressT = typename Storage<personality>::AddressT;
/// Mutates @c target such that @c source & @c source_mask replaces the bits that currently start
/// at @c shift bits from least significant. Subsequently ensures @c target is constrained by the
/// Mutates @c target such that @c source replaces the @c length bits that currently start
/// at bit @c shift . Subsequently ensures @c target is constrained by the
/// applicable @c memory_mask.
template <int shift, uint8_t source_mask = 0xff> void install_field(AddressT &target, uint8_t source) {
template <int shift, int length = 8> void install_field(AddressT &target, uint8_t source) {
static_assert(length > 0 && length <= 8);
constexpr auto source_mask = (1 << length) - 1;
constexpr auto mask = AddressT(~(source_mask << shift));
target = (
(target & mask) |
@ -491,6 +493,17 @@ template <Personality personality> struct Base: public Storage<personality> {
int sprite_height_ = 8;
// Programmer-specified addresses.
//
// The TMS and descendants combine various parts of the address with AND operations,
// e.g. the fourth byte in the pattern name table will be at `pattern_name_address_ & 4`;
// ordinarily the difference between that and plain substitution is invisible because
// the programmer mostly can't set low-enough-order bits. That's not universally true
// though, so this implementation uses AND throughout.
//
// ... therefore, all programmer-specified addresses are seeded as all '1's. As and when
// actual addresses are specified, the relevant bits will be substituted in.
//
// Cf. install_field.
AddressT pattern_name_address_ = memory_mask(personality); // Address of the tile map.
AddressT colour_table_address_ = memory_mask(personality); // Address of the colour map (if applicable).
AddressT pattern_generator_table_address_ = memory_mask(personality); // Address of the tile contents.