Spectrum ROM: Routine at 37AA

The address of this routine is found in the table of addresses. It is called via the calculator literal +20 by the routines at DRAW and tan. It is also called indirectly via fp_calc_2.

This subroutine handles the function COS X and returns a 'last value' 'that is an approximation to COS X.

The subroutine uses the expression COS X=SIN (πW/2), where -1<=W<=1.

In deriving W for X the subroutine uses the test result obtained in the previous subroutine and stored for this purpose in mem-0. It then jumps to the sin subroutine, entering at C_ENT, to produce a 'last value' of COS X.

The address of this routine is found in the table of addresses. It is called via the calculator literal +20 by the routines at DRAW and tan. It is also called indirectly via fp_calc_2. This subroutine handles the function COS X and returns a 'last value' 'that is an approximation to COS X. The subroutine uses the expression COS X=SIN (πW/2), where -1<=W<=1. In deriving W for X the subroutine uses the test result obtained in the previous subroutine and stored for this purpose in mem-0. It then jumps to the sin subroutine, entering at C_ENT, to produce a 'last value' of COS X.
cos	37AA	RST $28	X
	37AB	DEFB $39	get_argt: V
	37AC	DEFB $2A	abs: ABS V
	37AD	DEFB $A1	stk_one: ABS V, 1
	37AE	DEFB $03	subtract: ABS V-1
	37AF	DEFB $E0	get_mem_0: ABS V-1, (1/0)
	37B0	DEFB $00	jump_true to C_ENT: ABS V-1=W
	37B1	DEFB $06	jump_true to C_ENT: ABS V-1=W
If the jump was not made then continue.
	37B2	DEFB $1B	negate: 1-ABS V
	37B3	DEFB $33	jump to C_ENT: 1-ABS V=W
	37B4	DEFB $03	jump to C_ENT: 1-ABS V=W