問題入下:
A hypothetical single instruction computer SIC has only one instruction;
subtract and branch if negative, or sbn for short. The sbn instruction
has three operands, each consisting of the address of a word in memory:
sbn a,b,c # Mem[a]=Mem[a]-Mem[b]; if(Mem[a]<0) go to c
This instruction will subtract the number in memory location b from the
number in memory location a and place the result back in a, overwriting
the previous value. If the result is greater than or equal to 0, the
computer will take its next instruction from memory location just after
the current instruction. If the result is less than 0, the next instruction
is taken from memory location c. SIC has no registers and no instructions
other than sbn.
Although it has only one instruction, SIC can imitate many of the operations
of more complex instruction sets by using clever sequences of sbn instruction.
For example, here is a program to copy a number from location a to location b:
Start Sbn temp, temp,. +1 # sets temp to zero
Sbn temp, a,. +1 # sets temp to -a
Sbn b, b,. +1 # sets b to zero
Sbn b, temp,. +1 # sets b to temp which is a
In the program above, the notation "+1" means "the address after this one," so
that each instruction in this program goes on to the next in sequence whether
or not the result is negative. We assume temp to be the address of a apare
memory word that can be used for temporary result.
Please write SIC program to multiply a by b, putting the result in c.
請問大大們，這題要如何創造出乘法指令??
感謝!!
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