Emulating ‘MUL’

1. Emulating ‘MUL’ How multiplication of unsigned integers can be performed in software

2. Hardware support absent? • The earliest microprocessors (from Intel, Zilog, and others) did not implement the multiplication instructions – that operation would have had to be done in software • Even with our Core-2 Quad processor, there is an ultimate limit on the sizes of integers that can be multiplied using the processor’s built-in ‘multiply’ instructions

3. Multiplying with Base Ten • Here’s how we learn to multiply multi-digit integers using ordinary ‘decimal’ notation 8765  multiplicand x 4321  multiplier ------------------ 8765  partial product 17530  partial product 26295  partial product + 35060  partial product ------------------ 37873565  product

4. Analogy with Base Ten • When multiplying multi-digit integers using ‘binary’ notation, we apply the same idea 1001  multiplicand (=9) x 1011  multiplier (=11) ------------------ 1001  partial product 1001  partial product 0000  partial product + 1001  partial product ------------------ 1100011  product (=99)

5. Some observations… • With ‘binary’ multiplication of two N-digit values, the product can require 2N-digits • Each ‘partial product’ is either zero or is equal to the value of the ‘multiplicand’ • Succeeding partial products are ‘shifted’ • So if we want to ‘emulate’ multiplication of unsigned binary integers using software, we must implement these observations

6. 8-bit case • The smallest case to consider is using the ‘mul’ instruction to compute the product of 8-bit values, say in registers AL and BL .section .data number1: .byte 100 number2: .byte 200 product: .word 0 .section .text mov number1, %al mov number2, %bl mul %bl mov %ax, product

7. Doing it by hand 100 = 0x64 = 01100100 (binary) 01100100 AL 200 = 0xC8 = 11001000 (binary) 11001000 BL 00000000 (BL x 0) 00000000 (BL x 0) 11001000 (BL x 1) 00000000 (BL x 0) 00000000 (BL x 0) 11001000 (BL x 1) 11001000 (BL x 1) + 00000000 (BL x 0) ------------------------------------- 0100111000100000 20000 = 0x4E20 = 0100111000100000 (binary) 01001110 00100000 AX

8. Using x86 instructions .section .text softmulb: push %rcx # save caller’s count-register sub %ah, %ah # zero-extend AL to (CF:AH:AL) mov $9, %rcx # number of bits in (CF:AH) nxbit8: rcr $1, %ax # next multiplier-bit to Carry-Flag jnc noadd # skip addition if CF-bit is zero add %bl, %ah # else add the multiplicand noadd8: loop nxbit8 # go back to shift in next CF-bit sub %ah, %cl # set CF-bit if 8-bits exceeded pop %rcx # recover caller’s count-register ret

9. Visual depiction ROR $1, %AX CF AH AL 0 00000000 multiplier 17-bit value gets ‘rotated’ 1-place to the right BL multiplicand then multiplicand is added to AH (unless CF =0)

10. Exhaustive testing • We can insert ‘inline assembly language’ in a C++ program to construct a loop that checks our software multiply operation against every case of the CPU’s ‘mul’ • Our test-program is names ‘multest.cpp’ • You can compile it like this: $ g++ multest.cpp softmulb.s -o multest

11. In-class exercises • Can you write a ‘software’ emulation for the CPU’s 16-bit multiply operation? mul %bx • Can you write a ‘software’ emulation for the CPU’s 32-bit multiply operation? mul %ebx