JAL instruction has incorrectly restricted immediate

[erikeidt]

JAL instruction has incorrectly restricted immediate

[erikeidt]

	.text 0x00400000
	jal x0, label2

	.text 0x00408080

label1:
	lw	x1, 0(x1)

	.text 0x004085e8
label2:
	jal	x1, label1
	lw	x2, 0(x2)

[erikeidt]

This first branch, from 0x40 0000 to 0x40 85e8, is well within the 20-bit immediate range of the JAL U-Type instruction.

[TheThirdOne]

Thanks for the report. This is a problem in the assembler; the instruction encoding for jal is J-type as it should be.

The relevant lines in the assembler are

rars/rars/ProgramStatement.java

Lines 244 to 247 in 9ff2b88

	if (address >= (1 << 11) || address < -(1 << 11)) {
	errors.add(new ErrorMessage(this.sourceProgram, this.sourceLine, 0,
	"Jump target word address beyond 12-bit range"));
	return;

It should be possible to fix this simply by changing those lines to reflect the 20 bit range, but finding that raised some questions about by jalr isn't handled nearby so I am going to do some more investigation.

[TheThirdOne]

bd77755, fixes the size checks for jal and branches as it turns out >12bit offsets are allowed.

I would appreciate it if you confirmed that it now works as expected.

[erikeidt]

I tried the latest binary, and, it looks good to me — the reported problem is gone. Thanks, Erik From: Benjamin Landers [mailto:[email protected]] Sent: Tuesday, April 9, 2019 8:57 PM To: TheThirdOne/rars Cc: Erik Eidt; Author Subject: Re: [TheThirdOne/rars] JAL instruction has incorrectly restricted immediate (#19) bd77755<bd77755>, fixes the size checks for jal and branches as it turns out >12bit offsets are allowed. I would appreciate it if you confirmed that it now works as expected. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub<#19 (comment)>, or mute the thread<https://github.com/notifications/unsubscribe-auth/AAf6rADZMLHBxQ1kZ7mbqaHGSCyOMk-kks5vfWEkgaJpZM4clF7m>.