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Make an attempt at DIVS timing.

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This commit is contained in:
Thomas Harte 2022-05-27 14:56:04 -04:00
parent f599a78cad
commit 512cd333e5
1 changed files with 47 additions and 8 deletions
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55
Processors/68000Mk2/Implementation/68000Mk2Implementation.hpp
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@ -2522,13 +2522,13 @@ void ProcessorBase::did_bit_op(int bit_position) {
}
template <bool did_overflow> void ProcessorBase::did_divu(uint32_t dividend, uint32_t divisor) {
if(did_overflow) {
dynamic_instruction_length_ = 3; // Just a quick nn n, and then on to prefetch.
if(!divisor) {
dynamic_instruction_length_ = 4; // nn nn precedes the usual exception activity.
return;
}
if(!divisor) {
dynamic_instruction_length_ = 4; // nn nn precedes the usual exception activity.
if(did_overflow) {
dynamic_instruction_length_ = 3; // Just a quick nn n, and then on to prefetch.
return;
}
@ -2561,16 +2561,55 @@ template <bool did_overflow> void ProcessorBase::did_divu(uint32_t dividend, uin
}
}
template <bool did_overflow> void ProcessorBase::did_divs(int32_t, int32_t) {
// TODO: calculate cost.
}
#define convert_to_bit_count_16(x) \
x = ((x & 0xaaaa) >> 1) + (x & 0x5555); \
x = ((x & 0xcccc) >> 2) + (x & 0x3333); \
x = ((x & 0xf0f0) >> 4) + (x & 0x0f0f); \
x = ((x & 0xff00) >> 8) + (x & 0x00ff);
template <bool did_overflow> void ProcessorBase::did_divs(int32_t dividend, int32_t divisor) {
// The route to spotting divide by 0 is just nn nn.
if(!divisor) {
dynamic_instruction_length_ = 4; // nn nn precedes the usual exception activity.
return;
}
// It's either five or six microcycles to get into the main loop, depending
// on dividend sign.
dynamic_instruction_length_ = 5 + (dividend < 0);
if(did_overflow) {
return;
}
// There's always a cost of four microcycles per bit, plus an additional
// one for each that is non-zero.
//
// The sign bit does not count here; it's the low fifteen bits that matter
// only, in the unsigned version of the result.
dynamic_instruction_length_ += 60;
int result_bits = abs(dividend / divisor) & 0x7fff;
convert_to_bit_count_16(result_bits);
dynamic_instruction_length_ += result_bits;
// Determine the tail cost; a divisor of less than 0 leads to one exit,
// a divisor of greater than zero makes the result a function of the
// sign of the dividend.
//
// In all cases, this is counting from 'No more bits' in the Yacht diagram.
if(divisor < 0) {
dynamic_instruction_length_ += 4;
return;
}
if(dividend < 0) {
dynamic_instruction_length_ += 5;
} else {
dynamic_instruction_length_ += 3;
}
}
template <typename IntT> void ProcessorBase::did_mulu(IntT multiplier) {
// Count number of bits set.
convert_to_bit_count_16(multiplier);