Propeller Manual

CMPSUB – Assembly Language Reference
CMPSUB
Instruction: Compare two unsigned values and subtract the second if it is lesser or equal.
CMPSUB Value1, 〈#〉 Value2
Result: Optionally, Value1 = Value1 – Value2, and Z and C flags = comparison results.
•
Value1 (d-field) is the register containing the value to compare with that of Value2 and
is the destination in which to write the result if a subtraction is performed.
•
Value2 (s-field) is a register or a 9-bit literal whose value is compared with and
possibly subtracted from Value1.
Opcode Table:
–INSTR– ZCRI –CON– –DEST– –SRC– Z Result C Result Result Clocks
111000 001i 1111 ddddddddd sssssssss
D = S Unsigned (D => S) Written 4
Concise Truth Table:
In Out
Destination
1
Source
1
Z C Effects Destination Z C
$0000_0003; 3 $0000_0002; 2 - -
wz wc
$0000_0001; 1 0 1
$0000_0003; 3 $0000_0003; 3 - -
wz wc
$0000_0000; 0 1 1
$0000_0003; 3 $0000_0004; 4 - -
wz wc
$0000_0003; 3 0 0
1
Both Source and Destination are treated as unsigned values.
Explanation
CMPSUB compares the unsigned values of Value1 and Value2, and if Value2 is equal to or less
than Value1 then it is subtracted from Value1.
If the
WZ effect is specified, the Z flag is set (1) if Value1 equals Value2. If the WC effect is
specified, the C flag is set (1) if a subtraction is possible (Value1 is equal to or greater than
Value2). The result, if any, is written to Value1 unless the
NR effect is specified.
Page 276 · Propeller Manual v1.1