83Plus:Ports:21

This port only exists as a distinct port on the TI-83 Plus Silver Edition, the TI-84 Plus, and the TI-84 Plus Silver Edition. On the standard TI-83 Plus, it acts as a shadow of port 01.

This port is protected, which means user programs cannot ordinarily write to it directly.

Synopsis

Port Number: 21h

Function: Hardware Type / RAM Protection

This port is used as a flag to identify the hardware type (TI-84+ basic versus SE), as well as controlling the RAM execution protection.

Read Values

• Bit 0: 1 on either the 83+ SE or the 84+ SE, 0 on the 84+ basic.
• Bit 1: Unknown -- always set to zero
• Bits 4-5: current RAM protection mode (see below)

Write Values

• Bits 0, 1, 4, 5: set new values for these bits.
• Bits 2, 3, 6, 7: ignored

Comments

This port does not exist on the standard 83+. See port 2.

The value of this port is initialized by the boot code to either 01 (TI-83+ SE, TI-84+ SE) or 00 (TI-84+ basic.) Bit 0 can thus be used to determine the hardware type (in fact, the OS checks both bits 0 and 1, by ANDing the value with 3; it may be a good idea for user programs to do the same.)

RAM protection modes

Which RAM addresses are executable is determined by ports 25 and 26. The high nibble of port 21 controls how frequently the RAM protection loops. It will loop every 2^(value+1) pages. So, 0 loops every 2 pages, and 3 loops every 16 pages (in other words, not at all).

With the default values in ports 25 and 26:

• Mode 0 (the default): Execution is allowed on pages 81h, 83h, 85h, 87h, 89h, 8Bh, 8Dh, and 8Fh.
• Mode 1: Execution is allowed on pages 81h, 82h, 85h, 86h, 89h, 8Ah, 8Dh, and 8Eh.
• Mode 2: Execution is allowed on pages 81h, 82h, 89h, and 8Ah.
• Mode 3: Execution is allowed on pages 81h and 82h only.

(If you ever attempt to execute code from a "disallowed" page, the calculator resets.)

Example

	in a,(2)
	ld b,a
	and 80h
	jr z,TI83p_BE
	in a,(21h)
	and 3
	jr z,TI84p_BE
	bit 5,b
	jr z,TI83p_SE
	; calculator is an 84+ SE

TI84p_BE:
	; calculator is an 84+ BE

TI83p_SE:
	; calculator is an 83+ SE

TI83p_BE:
	; calculator is an 83+ BE