Signed vs unsigned behavior on 8-bit MCU in Embedded C - Visual Side-by-Side Comparison

Concept Flow - Signed vs unsigned behavior on 8-bit MCU

Start with 8-bit variable

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Assign value (signed or unsigned)

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Store in 8 bits

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Interpret bits as signed or unsigned

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Perform arithmetic or comparison

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Result depends on signedness

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Use result in program

This flow shows how an 8-bit variable stores values and how interpreting bits as signed or unsigned changes the result.

Execution Sample

Embedded C

unsigned char u = 250;
signed char s = 250;

printf("u = %u\n", u);
printf("s = %d\n", s);

This code assigns 250 to both unsigned and signed 8-bit variables and prints their values.

Execution Table

Step	Variable	Value Assigned	Stored Bits (8-bit)	Interpreted Value	Print Output
1	u (unsigned char)	250	11111010	250	u = 250
2	s (signed char)	250	11111010	-6	s = -6
3	End	-	-	-	Execution ends

💡 Execution stops after printing both values.

Variable Tracker

Variable	Start	After Assignment	Final
u (unsigned char)	undefined	250	250
s (signed char)	undefined	250 (stored as 11111010)	-6 (interpreted signed)

Key Moments - 3 Insights

Why does assigning 250 to a signed 8-bit variable show -6 when printed?

Because 8-bit signed variables use two's complement. The bits 11111010 represent -6 in signed form, as shown in execution_table row 2.

Why does the unsigned variable print 250 correctly?

Unsigned variables interpret bits as positive numbers only, so 11111010 is 250 decimal, as seen in execution_table row 1.

What happens if arithmetic is done on signed vs unsigned variables?

Arithmetic results depend on interpretation: signed variables can represent negative numbers, unsigned cannot, affecting overflow and comparisons.

Visual Quiz - 3 Questions

Look at the execution_table, what is the interpreted value of the signed char variable after assignment?

A 250

B -6

C 6

D -250

💡 Hint

Check execution_table row 2 under 'Interpreted Value' column.

At which step does the unsigned variable get assigned its value?

A Step 2

B Step 3

C Step 1

D No assignment

💡 Hint

Look at execution_table row 1 under 'Variable' and 'Value Assigned'.

If the signed char variable was assigned 130 instead of 250, what would be its interpreted value?

A -126

B 130

C -130

D 126

💡 Hint

Signed 8-bit range is -128 to 127; 130 stored as bits wraps to negative number similar to 250 case.

Concept Snapshot

Signed vs unsigned 8-bit variables store the same bits but interpret them differently.
Unsigned chars hold 0 to 255.
Signed chars hold -128 to 127 using two's complement.
Assigning values >127 to signed chars wraps to negative numbers.
Printing must match variable type to show correct value.

Full Transcript

This lesson shows how 8-bit microcontrollers store values in variables. When you assign a number like 250 to an unsigned 8-bit variable, it stores the bits 11111010 and prints 250. But if you assign 250 to a signed 8-bit variable, it stores the same bits but interprets them as -6 because signed 8-bit uses two's complement. This difference affects how arithmetic and comparisons work. Always remember to use the correct type and print format to see the expected values.