The Missing Instruction That Crippled Early CPUs

Comments · 426

• TC

  @tcmxiyw · 1 month ago

  Back in the day one of my colleagues complained to me that he had installed a coprocessor (80287?) and it disappointed him that he could see no increase in performance. I suggested that he should let his compiler know that the coprocessor was available. The next day he told me that it was like I had given him a supercomputer.

  54

• MU

  @mutant0407 · 1 month ago (edited)

  There are some very elegant multiply algorithms that work using precalculated tables of squares. This reduces the multiplication of two signed numbers to an addition, three array fetches, two subtracts and a final right shift, using AxB = ((A+B)^2-A^2 - B^2) / 2 . These are vastly quicker than traditional shift and add.<br>We found this algorithm in the Z80 port of the Elite space flight sim game written by Ricardo Pinto et al in 1985, which had 3d wireframe graphics.<br><br>The 8086 hardware integer multiply instruction was actually present on the core processor without an 87, but was actually slower than some of these software algorithms.

  68

• PI

  @Pihlalorjoone · 1 month ago

  My very first routine I wrote for the 6502 was a 16x16 bit multiplication. I loved it when I got it to work. And it was soo fast!

  69

• MU

  @MultiPetercool · 1 month ago

  I started on 6502. I love it because it is the ultimate learning experience for a programmer. <br><br>Compilers are great, but hand assembling instructions and poking them into RAM creates an appreciation for exactly how hardware works. <br><br>A good programmer needs to understand the limitations of the hardware.

  12

• CA

  @carlosanvito · 1 month ago

  I lived through that period and specifically added the 8087 math co-pro to the Compaq 8086 machine were were ordering to perform engineering calculations. &nbsp;That&apos;s how it was done then. &nbsp;As a coda, when that machine was sent to pasture, I removed the gold-plated 8087 and kept it as a fond memento.

  5

• MH

  @mheermance · 1 month ago (edited)

  This was true for most minicomputers and microprocessors into the early 80s. For example the 74LS181 ALU chip lacked a multiply operation and it was used in minicomputers of that era. Loop unrolling can make it faster if you have the RAM. The hardware multiply instruction is slower than others because there&apos;s no free lunch and multiplication is intrinsically more expensive than addition.

  7

• HA

  @HarveyIng · 1 month ago

  In 1970, we designed a hardware 24-bit multiplier for a signal processor. It is housed on a 3x5 modules using then advanced MSI integrated circuits. We did not use the standard shift and add algorithm but a much faster algorithm that add partial sums. It performed a 24-bit multiply in two 5MHz cycles.

  1

• HO

  @HotspotsSoutheast · 1 month ago

  I started programming in 1979 on circuit board computers. I programmed in 6502, 6809 in hexadecimal, no compiler. My favorite CPU was the MC6809 on the Color Computer. It had things none of the other CPUs had. It had two 8-bit accumulators that could be multiplied and the result was 16-bit. It had true relative branching for the entire memory range. It had optimized branching to reduce the number of bytes required to store the distance. I knew every single operation for the 6809 including the ones that were undocumented. I knew what those undocumented operations did and used them in my code. I even rewrote the disk operating system that Microsoft wrote and served a few hundred bytes, enough to add more BASIC commands.

  18

• CI

  @cigmorfil4101 · 4 weeks ago

  The algorithm to calculate square roots works extremely well in binary - more so than when calculating by hand in decimal.

• HC

  @HCherns · 1 month ago

  I recall that the CDC 6400, smaller brother to the supercomputer 6600 (what later we would call RISC machines), had no integer multiply op code. But one could convert &nbsp;the two numbers to floating-point, do a floating-point multiply, and turn the product into a normalized integer faster than most 360s could do an integer multiply.

  1

• NO

  @NoOneYouKnow2025-m8c · 1 month ago

  My first computer, the Radio Shack Color Computer, had a hardware multiply instruction as standard equipment.

• SL

  @slvsfr · 1 month ago

  My first computer was a Rockwell AIM 65, a one MHz 6502. &nbsp;No assembler, but a sort of neumatic entry. &nbsp;Calculation branches was a thing, A great vehicle for learning machine code and hardware interface.