I was referring to LODSB/W (x86) which is quite useful for processing arrays.
https://retrocomputing.stackexchange.com/questions/4744/how-...
Repeat is done by decrementing PC by 2 and re-loading whole instruction in a loop. 21 cycles per byte copied :o
To be fair Intel did same fail implementation of REP MOVSB/MOVSW in 8088/8086 reloading whole instruction per iteration, REP MOVSW is ~14 cycles/byte 8088 (9+27/rep) and ~9 cycles/byte 8086 (9+17/rep), ~same cost as non REP versions (28 and 18). NEC V20/V30 improved by almost 2x to 8 cycles/byte V20 or unaligned V30 (11+16/rep) and 4 cycles/byte on fully aligned access V30 (11+8/rep) with non REP cost being 19 and 11 respectively. V30 pretty much matched Intel 80186 4 cycles/byte (8+8/rep, 9 non rep). 286 was another jump to 2 cycles/byte (5+4/rep). 386 same speed, 486 much slower for small rep counts, under a cycle for big rep movsd. Pentium up to 0.31 cycles per byte, MMX 0.27 cycle/byte (http://www.pennelynn.com/Documents/CUJ/HTML/14.12/DURHAM1/DU...), then 2009 AVX doing block moves at full L2 cache speed and so on.
In 6502 corner there was nothing until 1986 WDC W65C816 Move Memory Negative (MVN), Move Memory Positive (MVP) 7 cycles/byte. Slower than unrolled code, 2x slower than unrolled code using 0 page. Similar bad implementation (no loop buffer) re-fetching whole instruction every iteration.
1987 NEC TurboGrafx-16/PC Engine 6502 clone by HudsonSoft HuC6280 Transfer Alternate Increment (TAI), Transfer Increment Alternate (TIA), Transfer Decrement Decrement (TDD), Transfer Increment Increment (TII) theoretical 6 cycles/byte (17+6rep). I saw one post long time ago claiming block transfer throughput of ~160KB/s on a 7.16 MHz NEC manufactured TurboGrafx-16 (hilarious 43 cycles/byte) so dont know what to think of it considering NEC V20 inside OG 4.77MHz IBM XT does >300KB/s.
CPU / Instruction Cycles per Byte
Z80 LDIR 8-bit 21
8088 MOVSW 8bit ~14
6502 LDA/STA 8bit ~14
8086 MOVSW ~9
NEC V20 MOVBKW 8bit ~8
W65C816 MVN/MVP 8bit ~7 block move
HuC6280 T[DIAX]/TIN 8bit ~6 block transfer instructions
80186 MOVSW 16bit ~4
NEC V30 MOVSW ~4
80286 MOVSW ~2
486 MOVSD <1
Pentium MOVSD ~0.31
Pentium MMX MOVSD ~0.27 http://www.pennelynn.com/Documents/CUJ/HTML/14.12/DURHAM1/DURT1.HTM CPU Cycles per theoretical minimum per byte for block move
Z80 instruction fetch 4 byte
Z80 data read/write 3 byte 6
80(1)88, V20 4 byte 8
80(1)86, V30 4 byte/word 4
80286, 80386 SX 2 byte/word 1
80386 DX 2 byte/word/dword 0.5
The microcode loop used by the 8086/8088 also had overhead, this was improved in the following generations. Then it became somewhat neglected since compilers / runtime libraries preferred to use sequences of vector instructions instead.
And with modern processors there are a lot of complications due to cache lines and paging, so there's always some unavoidable overhead at the start to align everything properly, even if then the transfer rate is close to optimal.
Moreover, the cache memories used with 286/386SX/386DX were normally write-through, which means that they shortened only the read cycles, not also the write cycles. Such caches were very effective to diminish the impact on performance of instruction fetching, but they brought little or no improvement to block transfers. The caches were also very small, so any sizable block transfer would flush the entire cache, then all transfers would be done at DRAM speed.
"12MHz/0 wait state with 100ns DRAM."
another https://theretroweb.com/chip/documentation/neat-6210302843ed...
"The processor can operate at 16MHz with 0.5-0.7 wait state memory accesses, using 100 nsec DRAMs. This is possible through the Page Interleaved memory scheme."
LDDR was the same but decremented HL and DE on each iteration instead.
There were versions for doing IN and OUT as well, and there was an instruction for finding a given byte value in a string, but I never used those so I don't recall the details.